/**
@page working_with_charts Working with Charts

@tableofcontents

This section explains how to work with some of the options and features of
@ref chart.h "The Chart object".

The majority of the examples in this section are based on a variation of the following program:

@dontinclude chart_working_with_example.c
@skip main
@until }

@image html chart_working.png



@section ww_charts_axes Chart Value and Category Axes

When working with charts it is important to understand how Excel
differentiates between a chart axis that is used for series categories and a
chart axis that is used for series values.

In the majority of Excel charts the X axis is the **category** axis and each
of the values is evenly spaced and sequential. The Y axis is the **value**
axis and points are displayed according to their value:

@image html chart_axes01.png

Excel treats these two types of axis differently and exposes different
properties for each. For example, here are the properties for a category axis:

@image html chart_axes02.png

Here are properties for a value axis:

@image html chart_axes03.png

As such, some of the `libxlsxwriter` axis properties can be set for a value
axis, some can be set for a category axis and some properties can be set for
both. The documentation calls out the type of axis to which functions apply
and the API emits a warning if the wrong type is used.

Some category axes, such as the one in the column chart example above, use
numbers as the categories. However, these numbers can't be treated like value
axes. In particular you can't use chart_axis_set_max() and
chart_axis_set_min() for category axes, as you can see in the respective
format dialogs above.

For a Bar chart the Category and Value axes are reversed:

@image html chart_axes04.png

A Scatter chart (but not a Line chart) has 2 value axes:

@image html chart_axes05.png

@note Date axes aren't supported yet.


@section chart_markers Working with Chart Markers

In Excel a chart marker is used to distinguish data points in a plotted
series. In general only Line and Scatter and Radar chart types use
markers. The libxlsxwriter chart types that can have markers are:

- #LXW_CHART_LINE
- #LXW_CHART_SCATTER
- #LXW_CHART_SCATTER_STRAIGHT
- #LXW_CHART_SCATTER_STRAIGHT_WITH_MARKERS
- #LXW_CHART_SCATTER_SMOOTH
- #LXW_CHART_SCATTER_SMOOTH_WITH_MARKERS
- #LXW_CHART_RADAR
- #LXW_CHART_RADAR_WITH_MARKERS

The chart types with `MARKERS` in the name have markers with default colors
and shapes turned on by default but it is possible to change them using the
functions shown below.

The `chart_series_set_marker_type()` function is used to specify the type of
the series marker:

@code
    chart_series_set_marker_type(series, LXW_CHART_MARKER_DIAMOND);
@endcode

@image html chart_marker1.png

The available marker types defined by #lxw_chart_marker_type are:

- #LXW_CHART_MARKER_AUTOMATIC
- #LXW_CHART_MARKER_NONE
- #LXW_CHART_MARKER_SQUARE
- #LXW_CHART_MARKER_DIAMOND
- #LXW_CHART_MARKER_TRIANGLE
- #LXW_CHART_MARKER_X
- #LXW_CHART_MARKER_STAR
- #LXW_CHART_MARKER_SHORT_DASH
- #LXW_CHART_MARKER_LONG_DASH
- #LXW_CHART_MARKER_CIRCLE
- #LXW_CHART_MARKER_PLUS

The `#LXW_CHART_MARKER_NONE` type can be used to turn off default markers:

@code
    chart_series_set_marker_type(series, LXW_CHART_MARKER_NONE);
@endcode

@image html chart_series_set_marker_none.png

The `#LXW_CHART_MARKER_AUTOMATIC` type is a special case which turns on a
marker using the default marker style for the particular series. If
automatic is on then other marker properties such as size, line or fill
cannot be set.

The `chart_series_set_marker_size()` function is used to specify the size of
the series marker:

@code
    chart_series_set_marker_type(series, LXW_CHART_MARKER_CIRCLE);
    chart_series_set_marker_size(series, 10);
@endcode

@image html chart_series_set_marker_size.png


The `chart_series_set_marker_line()` and `chart_series_set_marker_fill()`
functions can be used to set the line/border and fill properties of a chart
marker:

@code
    lxw_chart_line line = {.color = LXW_COLOR_BLACK};
    lxw_chart_fill fill = {.color = LXW_COLOR_RED};

    chart_series_set_marker_type(series, LXW_CHART_MARKER_SQUARE);
    chart_series_set_marker_size(series, 8);

    chart_series_set_marker_line(series, &line);
    chart_series_set_marker_fill(series, &fill);
@endcode

@image html chart_marker2.png

For more information on line/border and fill formatting see @ref chart_lines
and @ref chart_fills below.



@section chart_trendlines Working with Chart Trendlines

A trendline can be added to a chart series to indicate trends in the data
such as a moving average or a polynomial fit. The trendlines types are
shown in the following Excel dialog:

@image html chart_trendline0.png

The `chart_series_set_trendline()` function turns on these trendlines for
a data series:

@code
    chart = workbook_add_chart(workbook, LXW_CHART_LINE);
    series = chart_add_series(chart, NULL, "Sheet1!$A$1:$A$6");

    chart_series_set_trendline(series, LXW_CHART_TRENDLINE_TYPE_LINEAR, 0);
@endcode

@image html chart_trendline2.png

The `value` parameter corresponds to *order* for a polynomial trendline
and *period* for a Moving Average trendline. It both cases it must be >= 2.
The `value` parameter  is ignored for all other trendlines:

@code
    chart_series_set_trendline(series, LXW_CHART_TRENDLINE_TYPE_AVERAGE, 2);
@endcode

@image html chart_trendline3.png

The allowable values for the the trendline `type` are:

- #LXW_CHART_TRENDLINE_TYPE_LINEAR: Linear trendline.
- #LXW_CHART_TRENDLINE_TYPE_LOG: Logarithm trendline.
- #LXW_CHART_TRENDLINE_TYPE_POLY: Polynomial trendline. The `value`
  parameter corresponds to *order*.
- #LXW_CHART_TRENDLINE_TYPE_POWER: Power trendline.
- #LXW_CHART_TRENDLINE_TYPE_EXP: Exponential trendline.
- #LXW_CHART_TRENDLINE_TYPE_AVERAGE: Moving Average trendline. The `value`
  parameter corresponds to *period*.

Other trendline options, such as those shown in the following Excel
dialog, can be set using the functions described below.

@image html chart_trendline1.png

The `chart_series_set_trendline_forecast()` function sets the forward
and backward forecast periods for the trendline:

@code
    chart_series_set_trendline(series, LXW_CHART_TRENDLINE_TYPE_LINEAR, 0);
    chart_series_set_trendline_forecast(series, 0.5, 0.5);
@endcode

@image html chart_trendline4.png

@note This feature isn't available for Moving Average in Excel.

The `chart_series_set_trendline_equation()` function displays the
equation of the trendline on the chart:

@code
    chart_series_set_trendline(series, LXW_CHART_TRENDLINE_TYPE_LINEAR, 0);
    chart_series_set_trendline_equation(series);
@endcode

@image html chart_trendline5.png

@note This feature isn't available for Moving Average in Excel.

The `chart_series_set_trendline_r_squared()` function displays the
R-squared value for the trendline on the chart:

@code
    chart_series_set_trendline(series, LXW_CHART_TRENDLINE_TYPE_LINEAR, 0);
    chart_series_set_trendline_r_squared(series);
@endcode

@image html chart_trendline6.png

@note This feature isn't available for Moving Average in Excel.

The `chart_series_set_trendline_intercept()` function sets the Y-axis
intercept for the trendline:

@code
    chart_series_set_trendline(series, LXW_CHART_TRENDLINE_TYPE_LINEAR, 0);
    chart_series_set_trendline_equation(series);
    chart_series_set_trendline_intercept(series, 0.8);
@endcode

@image html chart_trendline7.png

As can be seen from the equation on the chart the intercept point
(when X=0) is the same as the value set in the equation.

@note The intercept feature is only available in Excel for Exponential,
      Linear and Polynomial trendline types.

The `chart_series_set_trendline_name()` function sets the name of the
trendline that is displayed in the chart legend. In the examples above
the trendlines are displayed with default names like "Linear (Series 1)"
and "2 per Mov. Avg. (Series 1)". If these names are too verbose or not
descriptive enough you can set your own trendline name:

@code
    chart_series_set_trendline(series, LXW_CHART_TRENDLINE_TYPE_LINEAR, 0);
    chart_series_set_trendline_name(series, "My trendline");
@endcode

@image html chart_trendline8.png

It is often preferable to turn off the trendline caption in the legend.
This is done in Excel by deleting the trendline name from the legend.
In libxlsxwriter this is done using the `chart_legend_delete_series()`
function to delete the zero based series numbers:

@code
    chart_series_set_trendline(series, LXW_CHART_TRENDLINE_TYPE_LINEAR, 0);

    // Delete the series name for the second series (=1 in zero base).
    // The -1 value indicates the end of the array of values.
    int16_t names[] = {1, -1};
    chart_legend_delete_series(chart, names);
@endcode

@image html chart_trendline9.png

The `chart_series_set_trendline_line()` function is used to set the line
properties of a trendline:

@code
    lxw_chart_line line = {.color     = LXW_COLOR_RED,
                           .dash_type = LXW_CHART_LINE_DASH_LONG_DASH};

    chart_series_set_trendline(series, LXW_CHART_TRENDLINE_TYPE_LINEAR, 0);
    chart_series_set_trendline_line(series, &line);
@endcode

@image html chart_trendline10.png

@note Trendlines cannot be added to series in a stacked chart, pie chart,
      doughnut chart or radar chart.

@section chart_error_bars Working with Chart Error Bars

Error bars can be added to a chart series to indicate error bounds in the
data.

The error bars can be vertical `y_error_bars` (the most common type) or
horizontal `x_error_bars` (for Bar and Scatter charts only). For
convenience these can be accessed from a series as follows:

@code
    chart_series_set_error_bars(series->y_error_bars, ...);

    chart_series_set_error_bars(series->x_error_bars, ...);
@endcode

The error bar properties that can be set in Excel are show in the following
dialog:

@image html chart_error_bars0.png

Some of these properties can be set using the functions discussed below.

The `chart_series_set_error_bars()` function sets the error bar type
and value associated with the type:

@code
    lxw_chart_series *series = chart_add_series(chart,
                                                "=Sheet1!$A$1:$A$5",
                                                "=Sheet1!$B$1:$B$5");

    chart_series_set_error_bars(series->y_error_bars,
                                LXW_CHART_ERROR_BAR_TYPE_STD_ERROR, 0);
@endcode

@image html chart_error_bars1.png

The error bar types that be used are:

- #LXW_CHART_ERROR_BAR_TYPE_STD_ERROR: Standard error.
- #LXW_CHART_ERROR_BAR_TYPE_FIXED: Fixed value.
- #LXW_CHART_ERROR_BAR_TYPE_PERCENTAGE: Percentage.
- #LXW_CHART_ERROR_BAR_TYPE_STD_DEV: Standard deviation(s).

@note Custom error bars are not currently supported.

All error bar types, apart from Standard error, should have a valid
value to set the error range:

@code
    chart_series_set_error_bars(series1->y_error_bars,
                                LXW_CHART_ERROR_BAR_TYPE_FIXED, 2);

    chart_series_set_error_bars(series2->y_error_bars,
                                LXW_CHART_ERROR_BAR_TYPE_PERCENTAGE, 5);

    chart_series_set_error_bars(series3->y_error_bars,
                                LXW_CHART_ERROR_BAR_TYPE_STD_DEV, 1);
@endcode

For the Standard error type the value is ignored.

The `chart_series_set_error_bars_direction()` function sets the
direction of the error bars:

@code
    chart_series_set_error_bars(series->y_error_bars,
                                LXW_CHART_ERROR_BAR_TYPE_STD_ERROR, 0);

    chart_series_set_error_bars_direction(series->y_error_bars,
                                          LXW_CHART_ERROR_BAR_DIR_PLUS);
@endcode

@image html chart_error_bars2.png

The valid directions are:

- #LXW_CHART_ERROR_BAR_DIR_BOTH: Error bar extends in both directions.
  The default.
- #LXW_CHART_ERROR_BAR_DIR_PLUS: Error bar extends in positive direction.
- #LXW_CHART_ERROR_BAR_DIR_MINUS: Error bar extends in negative direction.

The `chart_series_set_error_bars_endcap()` function sets the end cap
type for the error bars:

@code
    chart_series_set_error_bars(series->y_error_bars,
                                LXW_CHART_ERROR_BAR_TYPE_STD_ERROR, 0);

    chart_series_set_error_bars_endcap(series->y_error_bars,
                                       LXW_CHART_ERROR_BAR_NO_CAP);
@endcode

@image html chart_error_bars3.png

The valid values are:

- #LXW_CHART_ERROR_BAR_END_CAP: Flat end cap. The default.
- #LXW_CHART_ERROR_BAR_NO_CAP: No end cap.

The `chart_series_set_error_bars_line()` function sets the line
properties for the error bars:

@code
    lxw_chart_line line = {.color     = LXW_COLOR_RED,
                           .dash_type = LXW_CHART_LINE_DASH_ROUND_DOT};

    chart_series_set_error_bars(series->y_error_bars,
                                LXW_CHART_ERROR_BAR_TYPE_STD_ERROR, 0);

    chart_series_set_error_bars_line(series->y_error_bars, &line);
@endcode

@image html chart_error_bars4.png



@section chart_points Working with Chart Points

In general formatting is applied to an entire series in a chart. However, it
is occasionally required to format individual points within a series.

In Excel charts "points" have a different meaning depending on the type of
chart:

- Line/Scatter chart: points are used to reference individual markers.
- Bar/Column/Area charts: points are used to reference individual bars or areas.
- Pie/Doughnut charts: points are used to reference each segment.

The most common use case is to format segments of a pie chart like this
example:

@dontinclude chart_pie_colors.c
@skip include
@until close
@until }

@image html chart_points1.png

The #lxw_chart_point objects can be used to set the following properties for a
chart point:

- Line/Border
- Fill
- Pattern

These properties are explained in the Chart Formatting subsections below.

The points should be passed as a NULL terminated array of pointers to
#lxw_chart_point objects:

@code
    lxw_chart_point red_point   = {.fill = &red_fill  };
    lxw_chart_point green_point = {.fill = &green_fill};

    lxw_chart_point *points[] = {&green_point,
                                 &red_point,
                                 NULL}; // Indicates the end of the list.

    chart_series_set_points(series, points);
@endcode

You can skip points in the series that you don't want to format by passing a
zero-initialized #lxw_chart_point:

@code
    lxw_chart_point default_point = {0, 0, 0};


    lxw_chart_line line         = {.color = LXW_COLOR_BLACK};
    lxw_chart_fill red_fill     = {.color = LXW_COLOR_RED};
    lxw_chart_fill green_fill   = {.color = LXW_COLOR_GREEN};

    lxw_chart_point red_point   = {.fill = &red_fill,   .line = &line};
    lxw_chart_point green_point = {.fill = &green_fill, .line = &line};

    lxw_chart_point *points[] = {&red_point,
                                 &default_point,
                                 &green_point,
                                 &default_point,
                                 &red_point,
                                 NULL};

    chart_series_set_points(series, points);

    chart_series_set_marker_type(series, LXW_CHART_MARKER_SQUARE);
    chart_series_set_marker_size(series, 10);
@endcode

@image html chart_points2.png

The array of #lxw_chart_point objects pointers corresponds to the order of the
points in the series starting from the first point. However, it does not have
to extend to the entire range of the series. It can be NULL terminated at any
point in the series, such as in the previous example.

For Bar/Column/Area charts "points" refer to bars/areas within the chart:

@code
    lxw_chart_fill  red_fill      = {.color = LXW_COLOR_RED};
    lxw_chart_point red_point     = {.fill = &red_fill};
    lxw_chart_point default_point = {0, 0, 0};

    lxw_chart_point *points[] = {&default_point, &default_point, &red_point, NULL};

    chart_series_set_points(series2, points);
@endcode

@image html chart_points3.png


@section chart_labels Working with Chart Data Labels

Data labels can be added to a chart series to indicate the values of the
plotted data points. The functions described below turn on and set data
label properties.

The `chart_series_set_labels()` function is used to turn on data labels
for a chart series:

@code
    chart_series_set_labels(series);
@endcode

@image html chart_data_labels1.png

By default data labels are displayed in Excel with only the values shown:

@image html chart_data_labels2.png

However, it is possible to configure other display options, as shown
in the functions below.

The `chart_series_set_labels_options()` function is used to set the
parameters that are displayed in the series data label:

@code
    chart_series_set_labels(series);
    chart_series_set_labels_options(series, LXW_TRUE, LXW_TRUE, LXW_TRUE);
@endcode

@image html chart_data_labels3.png

The `chart_series_set_labels_separator()` function is used to change the
separator between multiple data label items. The default options is a comma
separator as shown in the previous example.

The available options are:

- #LXW_CHART_LABEL_SEPARATOR_SEMICOLON: semicolon separator.
- #LXW_CHART_LABEL_SEPARATOR_PERIOD: a period (dot) separator.
- #LXW_CHART_LABEL_SEPARATOR_NEWLINE: a newline separator.
- #LXW_CHART_LABEL_SEPARATOR_SPACE: a space separator.

For example:

@code
    chart_series_set_labels(series);
    chart_series_set_labels_options(series, LXW_TRUE, LXW_TRUE, LXW_TRUE);
    chart_series_set_labels_separator(series, LXW_CHART_LABEL_SEPARATOR_NEWLINE);
@endcode

@image html chart_data_labels4.png

The `chart_series_set_labels_position()` function sets the position of
the labels in the data series:

@code
    chart_series_set_labels(series);
    chart_series_set_labels_position(series, LXW_CHART_LABEL_POSITION_ABOVE);
@endcode

@image html chart_data_labels5.png

In Excel the allowable data label positions vary for different chart
types. The allowable, and default, positions are:

| Position                              | Line, Scatter | Bar, Column   | Pie, Doughnut | Area, Radar   |
| :------------------------------------ | :------------ | :------------ | :------------ | :------------ |
| #LXW_CHART_LABEL_POSITION_CENTER      | Yes           | Yes           | Yes           | Yes (default) |
| #LXW_CHART_LABEL_POSITION_RIGHT       | Yes (default) |               |               |               |
| #LXW_CHART_LABEL_POSITION_LEFT        | Yes           |               |               |               |
| #LXW_CHART_LABEL_POSITION_ABOVE       | Yes           |               |               |               |
| #LXW_CHART_LABEL_POSITION_BELOW       | Yes           |               |               |               |
| #LXW_CHART_LABEL_POSITION_INSIDE_BASE |               | Yes           |               |               |
| #LXW_CHART_LABEL_POSITION_INSIDE_END  |               | Yes           | Yes           |               |
| #LXW_CHART_LABEL_POSITION_OUTSIDE_END |               | Yes (default) | Yes           |               |
| #LXW_CHART_LABEL_POSITION_BEST_FIT    |               |               | Yes (default) |               |


The `chart_series_set_labels_leader_line()` function  is used to turn on
leader lines for the data label of a series. It is mainly used for pie
or doughnut charts:

@code
    chart_series_set_labels(series);
    chart_series_set_labels_leader_line(series);
@endcode

@note Even when leader lines are turned on they aren't automatically
      visible in Excel or XlsxWriter. Due to an Excel limitation
      (or design) leader lines only appear if the data label is moved
      manually or if the data labels are very close and need to be
      adjusted automatically.

The `chart_series_set_labels_legend()` function is used to set the
legend key for a data series:

@code
    chart_series_set_labels(series);
    chart_series_set_labels_legend(series);
@endcode

@image html chart_data_labels6.png

The `chart_series_set_labels_percentage()` function is used to turn on
the display of data labels as a percentage for a series. It is mainly
used for pie charts:

@code
    chart_series_set_labels(series);
    chart_series_set_labels_options(series, LXW_FALSE, LXW_FALSE, LXW_FALSE);
    chart_series_set_labels_percentage(series);
@endcode

@image html chart_data_labels7.png

The `chart_series_set_labels_num_format()` function is used to set the
number format for data labels:

@code
    chart_series_set_labels(series);
    chart_series_set_labels_num_format(series, "$0.00");
@endcode

@image html chart_data_labels8.png

The number format is similar to the Worksheet Cell Format num_format,
see `format_set_num_format()`.

The `chart_series_set_labels_font()` function is used to set the font
for data labels:

@code
    lxw_chart_font font = {.name = "Consolas", .color = LXW_COLOR_RED};

    chart_series_set_labels(series);
    chart_series_set_labels_font(series, &font);
@endcode

@image html chart_data_labels9.png

For more information see @ref chart_fonts below.

The `chart_series_set_labels_line()`, `chart_series_set_labels_fill()` and
`chart_series_set_labels_pattern()` functions can be used to set the
formatting for a data series using standard chart formatting structs:

@code
    lxw_chart_line line = {.color = LXW_COLOR_RED};
    lxw_chart_fill fill = {.color = LXW_COLOR_YELLOW};

    chart_series_set_labels_line(series, &line);
    chart_series_set_labels_fill(series, &fill);
@endcode

@image html chart_data_labels24.png


@subsection chart_custom_labels Custom Chart Data Labels

The `chart_series_set_labels_custom()` function is used to set the properties
of individual data labels in a series. The most common use for this is to set
custom text or number labels:

@code
    // Add the series data labels.
    chart_series_set_labels(series);

    // Create some custom labels.
    lxw_chart_data_label data_label1 = {.value = "Jan"};
    lxw_chart_data_label data_label2 = {.value = "Feb"};
    lxw_chart_data_label data_label3 = {.value = "Mar"};
    lxw_chart_data_label data_label4 = {.value = "Apr"};
    lxw_chart_data_label data_label5 = {.value = "May"};
    lxw_chart_data_label data_label6 = {.value = "Jun"};

    // Create an array of label pointers. NULL indicates the end of the array.
    lxw_chart_data_label *data_labels[] = {
        &data_label1,
        &data_label2,
        &data_label3,
        &data_label4,
        &data_label5,
        &data_label6,
        NULL
    };

    // Set the custom labels.
    chart_series_set_labels_custom(series, data_labels);
@endcode

@image html chart_data_labels18.png

As shown in the previous example the `chart_series_set_labels_custom()`
function takes a pointer to an array of #lxw_chart_data_label pointers. The
list should be `NULL` terminated. Any #lxw_chart_data_label items set to a
default initialization or omitted from the list will be assigned the default
data label value:

@code
    // Add the series data labels.
    chart_series_set_labels(series);

    // Create some custom labels.
    lxw_chart_data_label default_label = {0};
    lxw_chart_data_label data_label2   = {.value = "Feb"};
    lxw_chart_data_label data_label3   = {.value = "Mar"};
    lxw_chart_data_label data_label4   = {.value = "Apr"};


    // Create an array of label pointers. NULL indicates the end of the array.
    lxw_chart_data_label *data_labels[] = {
        &default_label,
        &data_label2,
        &data_label3,
        &data_label4,
        NULL
    };

    // Set the custom labels.
    chart_series_set_labels_custom(series, data_labels);
@endcode

@image html chart_data_labels19.png

The `value` element in the #lxw_chart_data_label struct should be a string, a
number as a string, or formula string that refers to a cell from which the
value will be taken:

@code
    // Add the series data labels.
    chart_series_set_labels(series);

    // Create some custom labels.
    lxw_chart_data_label data_label1 = {.value = "=Sheet1!$C$1"};
    lxw_chart_data_label data_label2 = {.value = "=Sheet1!$C$2"};
    lxw_chart_data_label data_label3 = {.value = "=Sheet1!$C$3"};
    lxw_chart_data_label data_label4 = {.value = "=Sheet1!$C$4"};
    lxw_chart_data_label data_label5 = {.value = "=Sheet1!$C$5"};
    lxw_chart_data_label data_label6 = {.value = "=Sheet1!$C$6"};

    // Create an array of label pointers. NULL indicates the end of the array.
    lxw_chart_data_label *data_labels[] = {
        &data_label1,
        &data_label2,
        &data_label3,
        &data_label4,
        &data_label5,
        &data_label6,
        NULL
    };

    // Set the custom labels.
    chart_series_set_labels_custom(series, data_labels);
@endcode

The `font` element in the #lxw_chart_data_label struct can be used to set the
font for the label in the data series (see @ref chart_fonts):

@code
    lxw_chart_font font = {.color = LXW_COLOR_RED};

    // Add the series data labels.
    chart_series_set_labels(series);

    // Create some custom labels.
    lxw_chart_data_label data_label1 = {.value = "=Sheet1!$C$1", .font = &font};
    lxw_chart_data_label data_label2 = {.value = "=Sheet1!$C$2", .font = &font};
    lxw_chart_data_label data_label3 = {.value = "=Sheet1!$C$3", .font = &font};
    lxw_chart_data_label data_label4 = {.value = "=Sheet1!$C$4", .font = &font};
    lxw_chart_data_label data_label5 = {.value = "=Sheet1!$C$5", .font = &font};
    lxw_chart_data_label data_label6 = {.value = "=Sheet1!$C$6", .font = &font};

    // Create an array of label pointers. NULL indicates the end of the array.
    lxw_chart_data_label *data_labels[] = {
        &data_label1,
        &data_label2,
        &data_label3,
        &data_label4,
        &data_label5,
        &data_label6,
        NULL
    };

    // Set the custom labels.
    chart_series_set_labels_custom(series, data_labels);
@endcode

@image html chart_data_labels20.png

The `hide` element in the #lxw_chart_data_label struct can be used to
hide/delete labels in a series. This can be useful if you want to highlight
one or more cells in the series, for example the maximum and the minimum:

@code
    // Add the series data labels.
    chart_series_set_labels(series);

    // Create some custom labels.
    lxw_chart_data_label hide = {.hide = LXW_TRUE};
    lxw_chart_data_label keep = {.hide = LXW_FALSE};

    // Create an array of label pointers. NULL indicates the end of the array.
    lxw_chart_data_label *data_labels[] = {
        &hide,
        &keep,
        &hide,
        &hide,
        &keep,
        &hide,
        NULL
    };

    // Set the custom labels.
    chart_series_set_labels_custom(series, data_labels);
@endcode

@image html chart_data_labels21.png


Standard chart formatting such as #lxw_chart_line, #lxw_chart_fill and
#lxw_chart_pattern can also be applied to custom data labels via
#lxw_chart_data_label:

@code
    // Set the border/line and fill for the data labels.
    lxw_chart_line line2 = {.color = LXW_COLOR_RED};
    lxw_chart_fill fill2 = {.color = LXW_COLOR_YELLOW};
    lxw_chart_line line3 = {.color = LXW_COLOR_BLUE};
    lxw_chart_fill fill3 = {.color = LXW_COLOR_GREEN};

    // Create some custom labels.
    lxw_chart_data_label data_label9_1 = {.value = "Amy", .line = &line3};
    lxw_chart_data_label data_label9_2 = {.value = "Bea"};
    lxw_chart_data_label data_label9_3 = {.value = "Eva"};
    lxw_chart_data_label data_label9_4 = {.value = "Fay"};
    lxw_chart_data_label data_label9_5 = {.value = "Liv"};
    lxw_chart_data_label data_label9_6 = {.value = "Una", .fill = &fill3};

    // Set the default formatting for the data labels in the series.
    chart_series_set_labels_line(series, &line2);
    chart_series_set_labels_fill(series, &fill2);

    // Create an array of label pointers. NULL indicates the end of the array.
    lxw_chart_data_label *data_labels9[] = {
        &data_label9_1,
        &data_label9_2,
        &data_label9_3,
        &data_label9_4,
        &data_label9_5,
        &data_label9_6,
        NULL
    };

    /* Set the custom labels. */
    chart_series_set_labels_custom(series, data_labels9);
@endcode

@image html chart_data_labels25.png


@section chart_formatting Chart Formatting

The following chart formatting properties can be set for any chart object that
they apply to (and that are supported by libxlsxwriter) such as chart lines,
column fill areas and other chart elements:

- Font
- Line/Border
- Fill
- Pattern

For example:

@code
    lxw_chart_line chart_line  = {.color = LXW_COLOR_BLUE};
    lxw_chart_line marker_line = {.color = LXW_COLOR_RED};
    lxw_chart_fill marker_fill = {.color = LXW_COLOR_YELLOW};

    chart_series_set_marker_type(series, LXW_CHART_MARKER_SQUARE);
    chart_series_set_marker_size(series, 5);

    chart_series_set_line(series, &chart_line);
    chart_series_set_marker_line(series, &marker_line);
    chart_series_set_marker_fill(series, &marker_fill);
@endcode

@image html chart_formatting2.png

These properties are explained in the subsections below.

@subsection chart_fonts Chart formatting: Fonts

Font properties can be set for several chart objects such as chart titles,
axis labels, and axis numbering.

A chart font #lxw_chart_font struct with default properties is:

@code
    lxw_chart_font font = {.name      = "Calibri",
                           .size      = 10,
                           .bold      = LXW_FALSE,
                           .italic    = LXW_FALSE,
                           .underline = LXW_FALSE,
                           .rotation  = 0,
                           .color     = LXW_COLOR_BLACK};
@endcode


The font properties are:

- `name`: Set the font name:

@code
    lxw_chart_font font = {.name = "Arial"};
@endcode

- `size`: Set the font size:

@code
    lxw_chart_font font = {.name = "Arial", .size = 11};
@endcode

- `bold`: Set the font bold property:

@code
    lxw_chart_font font = {.bold = LXW_TRUE};
@endcode

- `italic`: Set the font italic property:

@code
    lxw_chart_font font = {.italic = LXW_TRUE};
@endcode

- `underline`: Set the font underline property:

@code
    lxw_chart_font font = {.underline = LXW_TRUE};
@endcode

- `rotation`: Set the font rotation, angle, property in the range -90 to 90
deg:

@code
    lxw_chart_font font = {.rotation = 45};
@endcode

  This is useful for displaying axis data such as dates in a more compact
  format.

- `color`: Set the font color property. It can be a HTML style RGB color or a
  limited number of named colors (see @ref working_with_colors for more
  information):

@code
    lxw_chart_font font = {.color = LXW_COLOR_BLUE};
@endcode

Here is a longer example with several chart formats:


@dontinclude chart_fonts.c
@skip lxw_chart_font
@until chart_legend_set_font

@image html chart_fonts.png


@subsection chart_lines Chart formatting: Line

The line format is used to specify properties of line objects that appear in a
chart such as a plotted line on a chart or a border.

A chart line #lxw_chart_line struct with default properties is:

@code
    lxw_chart_line line = {.none      = LXW_FALSE,
                           .color     = LXW_COLOR_BLACK,
                           .width     = 2.25,
                           .dash_type = LXW_CHART_LINE_DASH_SOLID};
@endcode

The `none` member is uses to turn the line off (it is always on by default
except in Scatter charts). This is useful if you wish to plot a series with
markers without a line, or a column fill without a border:

@code
    lxw_chart_line line = {.none = LXW_TRUE};

    chart_series_set_line(series, &line);
    chart_series_set_marker_type(series, LXW_CHART_MARKER_AUTOMATIC);
@endcode

@image html chart_formatting3.png

The `color` member sets the color of the line. It can be a HTML style RGB
color or a limited number of named colors (see @ref working_with_colors for
more information):

@code
    lxw_chart_line line = {.color = 0xFF9900};

    chart_series_set_line(series, &line);
@endcode

@image html chart_formatting4.png


The `width` member sets the width of the line. It should be specified in
increments of 0.25 of a point as in Excel:

@code
    lxw_chart_line line = {.width = 4.25};

    chart_series_set_line(series, &line);
@endcode

@image html chart_formatting_width.png


The `dash_type` member sets the dash style of the line:

@code
    lxw_chart_line line = {.dash_type = LXW_CHART_LINE_DASH_DASH_DOT};

    chart_series_set_line(series, &line);
@endcode

@image html chart_formatting5.png

The following #lxw_chart_line_dash_type values are available. They are shown
in the order that they appear in the Excel dialog:

 Define                                | Excel name
 ------------------------------------- | ------------------
 LXW_CHART_LINE_DASH_SOLID             | Solid
 LXW_CHART_LINE_DASH_ROUND_DOT         | Round Dot
 LXW_CHART_LINE_DASH_SQUARE_DOT        | Square Dot
 LXW_CHART_LINE_DASH_DASH              | Dash
 LXW_CHART_LINE_DASH_DASH_DOT          | Dash Dot
 LXW_CHART_LINE_DASH_LONG_DASH         | Long Dash
 LXW_CHART_LINE_DASH_LONG_DASH_DOT     | Long Dash Dot
 LXW_CHART_LINE_DASH_LONG_DASH_DOT_DOT | Long Dash Dot Dot

The default line style is `#LXW_CHART_LINE_DASH_SOLID`.

More than one line property can be specified at a time:

@code
    lxw_chart_line line = {.color     = LXW_COLOR_RED,
                           .width     = 1.25,
                           .dash_type = LXW_CHART_LINE_DASH_SQUARE_DOT};
@endcode

@image html chart_formatting_multiple.png


@subsection chart_borders Chart formatting: Border

In Excel chart formatting the `border` property is a synonym for `line` when
the object being formatting also has a `fill` property.

Anywhere you see `border` in an Excel chart dialog you can use the equivalent
libxlsxwriter `line` function.


@subsection chart_fills Chart formatting: Fill

The fill format is used to specify properties of chart objects that internal
boundaries such as a column chart.


A chart fill #lxw_chart_fill struct with default properties is:

@code
    lxw_chart_fill fill = {.none         = LXW_FALSE,
                           .color        = LXW_COLOR_BLACK,
                           .transparency = 0};
@endcode


The none property is used to turn the fill property off (it is generally on by
default):

@code
    lxw_chart_line line = {.color = LXW_COLOR_BLACK};
    lxw_chart_fill fill = {.none  = LXW_TRUE};

    chart_series_set_line(series, &line);
    chart_series_set_fill(series, &fill);
@endcode

@image html chart_fill1.png

The `color` member sets the color of the fill area. It can be a HTML style RGB
color or a limited number of named colors (see @ref working_with_colors for
more information):

@code
    lxw_chart_fill fill = {.color = 0xFF9900};

    chart_series_set_fill(series, &fill);
@endcode

@image html chart_fill2.png

The `transparency` member sets the transparency of the solid fill color in the
integer range 1 - 100:

@code
    lxw_chart_fill fill = {.color        = LXW_COLOR_YELLOW,
                           .transparency = 50};

    chart_series_set_fill(series, &fill);
@endcode

@image html chart_fill3.png

The `fill` format is generally used in conjunction with a border which can be
set via the `line` format:

@code
    lxw_chart_line line = {.color = LXW_COLOR_BLACK};
    lxw_chart_fill fill = {.color = LXW_COLOR_RED};

    chart_series_set_line(series, &line);
    chart_series_set_fill(series, &fill);
@endcode

@image html chart_fill4.png


@subsection chart_patterns Chart formatting: Pattern

The pattern fill is used to specify pattern filled areas of chart objects such
as the interior of a column or the background of the chart itself.

A chart pattern #lxw_chart_pattern struct with default properties is:

@code
    lxw_chart_pattern pattern = {.type      = LXW_CHART_PATTERN_NONE,
                                 .fg_color  = LXW_COLOR_NONE,;
                                 .bg_color  = LXW_COLOR_WHITE};
@endcode

Where the members are:

- `pattern`: The pattern to be applied (required).
- `fg_color`: The foreground color of the pattern (required).
- `bg_color`: The background color (optional, defaults to white).


The foreground color, `fg_color`, is a required parameter and can be HTML
style RGB color or a limited number of named colors (see
@ref working_with_colors for more information):

The background color, `bg_color`, is optional and defaults to LXW_COLOR_WHITE.

For example:

@code
    lxw_chart_pattern pattern1 = {.type = LXW_CHART_PATTERN_SHINGLE,
                                  .fg_color = 0x804000,
                                  .bg_color = 0XC68C53};

    lxw_chart_pattern pattern2 = {.type = LXW_CHART_PATTERN_HORIZONTAL_BRICK,
                                  .fg_color = 0XB30000,
                                  .bg_color = 0XFF6666};

    chart_series_set_pattern(series1, &pattern1);
    chart_series_set_pattern(series2, &pattern2);

@endcode

@image html chart_pattern.png

The following patterns #lxw_chart_pattern_type can be applied:

- #LXW_CHART_PATTERN_PERCENT_5
- #LXW_CHART_PATTERN_PERCENT_10
- #LXW_CHART_PATTERN_PERCENT_20
- #LXW_CHART_PATTERN_PERCENT_25
- #LXW_CHART_PATTERN_PERCENT_30
- #LXW_CHART_PATTERN_PERCENT_40
- #LXW_CHART_PATTERN_PERCENT_50
- #LXW_CHART_PATTERN_PERCENT_60
- #LXW_CHART_PATTERN_PERCENT_70
- #LXW_CHART_PATTERN_PERCENT_75
- #LXW_CHART_PATTERN_PERCENT_80
- #LXW_CHART_PATTERN_PERCENT_90
- #LXW_CHART_PATTERN_LIGHT_DOWNWARD_DIAGONAL
- #LXW_CHART_PATTERN_LIGHT_UPWARD_DIAGONAL
- #LXW_CHART_PATTERN_DARK_DOWNWARD_DIAGONAL
- #LXW_CHART_PATTERN_DARK_UPWARD_DIAGONAL
- #LXW_CHART_PATTERN_WIDE_DOWNWARD_DIAGONAL
- #LXW_CHART_PATTERN_WIDE_UPWARD_DIAGONAL
- #LXW_CHART_PATTERN_LIGHT_VERTICAL
- #LXW_CHART_PATTERN_LIGHT_HORIZONTAL
- #LXW_CHART_PATTERN_NARROW_VERTICAL
- #LXW_CHART_PATTERN_NARROW_HORIZONTAL
- #LXW_CHART_PATTERN_DARK_VERTICAL
- #LXW_CHART_PATTERN_DARK_HORIZONTAL
- #LXW_CHART_PATTERN_DASHED_DOWNWARD_DIAGONAL
- #LXW_CHART_PATTERN_DASHED_UPWARD_DIAGONAL
- #LXW_CHART_PATTERN_DASHED_HORIZONTAL
- #LXW_CHART_PATTERN_DASHED_VERTICAL
- #LXW_CHART_PATTERN_SMALL_CONFETTI
- #LXW_CHART_PATTERN_LARGE_CONFETTI
- #LXW_CHART_PATTERN_ZIGZAG
- #LXW_CHART_PATTERN_WAVE
- #LXW_CHART_PATTERN_DIAGONAL_BRICK
- #LXW_CHART_PATTERN_HORIZONTAL_BRICK
- #LXW_CHART_PATTERN_WEAVE
- #LXW_CHART_PATTERN_PLAID
- #LXW_CHART_PATTERN_DIVOT
- #LXW_CHART_PATTERN_DOTTED_GRID
- #LXW_CHART_PATTERN_DOTTED_DIAMOND
- #LXW_CHART_PATTERN_SHINGLE
- #LXW_CHART_PATTERN_TRELLIS
- #LXW_CHART_PATTERN_SPHERE
- #LXW_CHART_PATTERN_SMALL_GRID
- #LXW_CHART_PATTERN_LARGE_GRID
- #LXW_CHART_PATTERN_SMALL_CHECK
- #LXW_CHART_PATTERN_LARGE_CHECK
- #LXW_CHART_PATTERN_OUTLINED_DIAMOND
- #LXW_CHART_PATTERN_SOLID_DIAMOND

@note If a pattern fill is used on a chart object it overrides the solid fill
      properties of the object.

@section ww_charts_limitations Chart Limitations

The following chart features aren't currently supported in libxlsxwriter but
will be in time. See the [GitHub Chart Feature Requests][1].

[1]: https://github.com/jmcnamara/libxlsxwriter/issues

- Secondary axis.
- Combined charts.
- Chartsheets.

If required these features are all available in the Perl and Python versions
of this library.


Next: @ref working_with_object_positioning

*/
