.. _lv_chart:

================ Chart (lv_chart)

Overview

Chart Widgets are used to visualize data.

Charts have:

0 or more background division lines (horizontal and vertical),
4 internal axes, any of which can be used to specify scale for a data series,
a point_count (default 10) that applies to all data series added,
0 or more data series (you can add or remove them at any time),
0 or more cursors (you can add or remove them at any time),
update mode (modifies behavior of :cpp:func:lv_chart_set_next_value if you use it).

Charts can:

display flexible axes with ticks and text
show precise locations of points or other locations on chart with cursors
show or hide individual data series
show or hide individual data points
show or hide cursors
scroll and zoom

Chart Basics

Each chart has the following attributes (over and above attributes found in :ref:all Widgets <base_widget>):

Type (governs how a chart's data series are drawn)

Can be LINE (default), BAR, STACKED, SCATTER, or NONE.
You can change the chart's type at any point during its life.

Horizontal and Vertical division lines

default 3 and 5 respectively
can be any non-negative value including 0

2 Y axes and 2 X axes (the latter are used with SCATTER charts)

All 4 axes come with each chart automatically (they do not have to be created).
Their default ranges are [0..100]. If you need a different range, set it before chart is drawn.
You "use" an axis by associating it with a data series, which happens when the data series is created (more on this below). More than one data series can be associated with each axis.

Point count (number of data points in all data series added to the chart)

default 10
If you provide your own data-value arrays, each array so provided must contain at least this number of values.
For LINE-, CURVE-, BAR-, STACKED-charts, this is the number of points on the X axis.
LINE-, CURVE-, BAR-, STACKED-charts require only one data-value array to supply Y-values for each data point.
For SCATTER charts, this is the number of scatter-points in the data series.
SCATTER charts have separate data-value arrays for both X-values and Y-values.

Any number of data series

After a chart is created, it initially contains no data series. You have to add them.
You can add and remove data series at any time during a chart's life.
When a data series is created, it comes with pre-allocated values array(s) based on its chart type and point_count. (All chart types use an array of Y-values. SCATTER-type charts also use an array of X-values.). All Y-values so allocated are set to :c:macro:LV_CHART_POINT_NONE, which causes that point to be hidden.
To get points to be drawn on the chart, you must set their Y-values to something other than :c:macro:LV_CHART_POINT_NONE.
You can hide a point by setting its Y-value to :c:macro:LV_CHART_POINT_NONE.
If desired, you can tell a data series to instead use a value array you provide. If you do:
- Pre-allocated value arrays are automatically freed.
- That data series will continue to use your array from that time onward.
- The values in your array must remain available through the life of that data series.
- You must ensure each array provided contains at least point_count int32_t elements.
- Management of the life any value arrays you provide is up to you.

Any number of cursors

After a chart is created, it initially contains no cursors. You have to add them if you want to use them.
You can add, show, hide or remove cursors at any time during a chart's life.

Update mode

:ref:See below <chart_update_modes>

Chart layers

When a chart is drawn, certain things appear on top of other things, in this order, from back to front:

The chart's background (with optional division lines)
Each data series:
- Earliest data series added appears on top.
- For a SCATTER chart, within each series, points later in the sequence will appear on top of points earlier in the sequence when there is overlap.
Each cursor (if there are any):
- The most recent cursor added appears on top.

.. _lv_chart_parts_and_styles:

Parts and Styles

:cpp:enumerator:LV_PART_MAIN The background of the chart. Uses the :ref:typical background <typical bg props> and line style properties (for division lines). Padding makes the series area smaller. For BAR and STACKED charts pad_column sets the space between bars in the same data series.
:cpp:enumerator:LV_PART_SCROLLBAR A scrollbar used if the chart is zoomed. See :ref:base_widget's documentation for details.
:cpp:enumerator:LV_PART_ITEMS Refers to the LINE or BAR data series.
- LINE chart: line properties are used by lines. width, height, bg_color and radius are used to set the appearance of points on the line.
- Bar chart: The typical background properties are used to style the bars. pad_column sets the space between columns in the same data series.
:cpp:enumerator:LV_PART_INDICATOR Refers to points on LINE- and SCATTER-charts (small circles or squares [with possibly-rounded corners]).
:cpp:enumerator:LV_PART_CURSOR Line properties are used to style cursors. width, height, bg_color and radius are used to set the appearance of the cursor's "point" showing its location. If either width or height are set to 0, only the cursor's lines are drawn.

.. _lv_chart_details:

Details

Chart type

A chart can be one of the following types:

:cpp:enumerator:LV_CHART_TYPE_NONE: Do not display any data. Can be used to hide chart's data.
:cpp:enumerator:LV_CHART_TYPE_LINE: Draw lines between data points. Data points can also be illustrated if their width, height, bg_color and radius styles (for :cpp:enumerator:LV_PART_ITEMS) are set and both width and height have non-zero values.
:cpp:enumerator:LV_CHART_TYPE_CURVE: Similar to the LINE type, but it draws Bezier curves between data points. LV_USE_VECTOR_GRAPHICS and a draw unit (e.g. VGLite, or ThorVG for software rendering) need to be enabled. It also supports the line_dash_gap/width style properties.
:cpp:enumerator:LV_CHART_TYPE_BAR: Draw individual vertical bars for each point in each series.
:cpp:enumerator:LV_CHART_TYPE_STACKED: Draw vertical stacked bars where multiple data series are displayed as segments within a single bar for each data point. Supports only positive values.
:cpp:enumerator:LV_CHART_TYPE_SCATTER: X/Y chart drawing point's and optionally lines between the points if line-width style values for :cpp:enumerator:LV_PART_ITEMS is a non-zero value, and the point's Y-value is something other than :c:macro:LV_CHART_POINT_NONE. (Drawing of individual points on a SCATTER chart can be suppressed if their Y-values are set to :c:macro:LV_CHART_POINT_NONE.)

Charts start their life as LINE charts. You can change a chart's type with :cpp:expr:lv_chart_set_type(chart, LV_CHART_TYPE_...).

Data series

You can add any number of data series to a chart by using

:cpp:expr:`lv_chart_add_series(chart, color, axis)`.

This allocates (and returns a pointer to) an :cpp:expr:lv_chart_series_t structure which remembers the color and axis you specified, and comes pre-allocated with an array of chart->point_cnt int32_t Y-values, all set to :c:macro:LV_CHART_POINT_NONE. (A SCATTER chart also comes with a pre-allocated array of the same number of X-values.)

axis specifies which axis is used to scale its values, and may be one of the following:

:cpp:enumerator:LV_CHART_AXIS_PRIMARY_Y: Left axis
:cpp:enumerator:LV_CHART_AXIS_SECONDARY_Y: Right axis
:cpp:enumerator:LV_CHART_AXIS_PRIMARY_X: Bottom axis
:cpp:enumerator:LV_CHART_AXIS_SECONDARY_X: Top axis

When adding a data series to a SCATTER chart, bit-wise OR your selected Y axis (primary or secondary) with one of the X-axis values.

If you wish to have the chart use your own Y-value array instead of the one provided, you can do so with

:cpp:expr:`lv_chart_set_series_ext_y_array(chart, series, value_array)`.

You are responsible for ensuring the array you provide contains at least chart->point_cnt elements in it.

value_array should look like this: int32_t * value_array[num_points]. Only the array's pointer is saved in the series so its contents need to remain available for the life of the series, i.e. the array needs to be global, static or dynamically allocated.

.. note:: Call :cpp:expr:lv_chart_refresh(chart) when a chart's data has changed to signal that the chart should be re-rendered next time a display refresh occurs. You do not need to do this if you are using the provided value array(s) and setting values with lv_chart_set_...value_...() functions. See below for more information about these functions.

A pointer to the Y-value array of a series can be obtained with :cpp:expr:lv_chart_get_series_y_array(chart, series). This is true whether you are using the provided Y-value array or provided your own.

For SCATTER-type charts,

:cpp:expr:lv_chart_set_series_ext_x_array(chart, series, value_array) and
:cpp:expr:lv_chart_get_series_x_array(chart, series)

can be used as well.

Modifying data

You have several options to set the Y-values for a data series:

Set the values programmatically in the array like ser1->points[3] = 7 and refresh the chart with :cpp:expr:lv_chart_refresh(chart).
Use :cpp:expr:lv_chart_set_series_value_by_id(chart, series, id, value) where id is the zero-based index of the point you wish to update.
Use :cpp:expr:lv_chart_set_next_value(chart, series, value). (See Update modes_ below.)
Set all points to a single Y-value with :cpp:expr:lv_chart_set_all_values(chart, series, value).

Use :c:macro:LV_CHART_POINT_NONE as value to make the library skip drawing that point, column, or scatter-point.

For SCATTER-type charts,

:cpp:expr:lv_chart_set_series_value_by_id2(chart, series, id, x_value, y_value) and
:cpp:expr:lv_chart_set_next_value2(chart, series, x_value, y_value)

can be used as well.

.. _chart_update_modes: .. _update modes:

Update modes

:cpp:func:lv_chart_set_next_value can behave in two ways depending on update mode:

:cpp:enumerator:LV_CHART_UPDATE_MODE_SHIFT: Shift old data to the left and add the new one to the right.
:cpp:enumerator:LV_CHART_UPDATE_MODE_CIRCULAR: Add the new data in circular fashion, like an ECG diagram.

The update mode can be changed with :cpp:expr:lv_chart_set_update_mode(chart, LV_CHART_UPDATE_MODE_...).

Number of points

The number of points in the series can be modified by :cpp:expr:lv_chart_set_point_count(chart, point_num). The default value is 10. Note: this affects the number of points processed when an external value array is assigned to a series, so you also need to be sure any external array so provided contains at least point_num elements.

Handling large numbers of points ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

On LINE charts, if the number of points is greater than the pixels horizontally, the Chart will draw only vertical lines to make the drawing of large amount of data effective. If there are, let's say, 10 points to a pixel, LVGL searches the smallest and the largest value and draws a vertical lines between them to ensure no peaks are missed.

Vertical range

You can specify the minimum and maximum values in Y-direction with :cpp:expr:lv_chart_set_axis_range(chart, axis, min, max). axis can be :cpp:enumerator:LV_CHART_AXIS_PRIMARY_Y (left Y axis) or :cpp:enumerator:LV_CHART_AXIS_SECONDARY_Y (right Y axis).

The value of the points will be scaled proportionally. The default range is: 0..100.

Division lines

The number of horizontal and vertical division lines can be modified by :cpp:expr:lv_chart_set_div_line_count(chart, hdiv_num, vdiv_num). The default settings are 3 horizontal and 5 vertical division lines. If there is a visible border on a side and no padding on that side, the division line would be drawn on top of the border and in this case it is not drawn so as not to hide the chart border.

Override default start point for series

If you want a plot to start from a point other than the default which is point[0] of the series, you can set an alternative index with the function :cpp:expr:lv_chart_set_x_start_point(chart, series, id) where id is the new zero-based index position to start plotting from.

Note that :cpp:enumerator:LV_CHART_UPDATE_MODE_SHIFT also changes the start_point.

Tick marks and labels

With the help of :ref:Scale <lv_scale>, vertical and horizontal scales can be added in a very flexible way. See the examples 2 <#axis-ticks-and-labels-with-scrolling>_ below to learn more.

Zoom

To zoom the chart all you need to do is wrap it in a parent container and set the chart's width and/or height to a larger value. Doing this will cause the chart to be scrollable in its parent --- the parent container provides the scrollable "view window".

Cursor

A new cursor is initially given position :c:macro:LV_CHART_POINT_NONE which causes it to be hidden. To show the cursor, its location must be set by you programmatically using one of the functions below.

You can hide a cursor without removing it from the chart by using :cpp:func:lv_chart_set_cursor_point by passing :c:macro:LV_CHART_POINT_NONE as the point id.

A cursor can be added with lv_chart_cursor_t * c1 = lv_chart_add_cursor(chart, color, dir);. The possible values of dir are the enumeration values of :cpp:type:lv_dir_t: LV_DIR_NONE/RIGHT/UP/LEFT/DOWN/HOR/VER/ALL or their bit-wise OR-ed values to tell the chart which direction(s) to draw its lines.

:cpp:expr:lv_chart_set_cursor_pos(chart, cursor, &point) sets the position of the cursor to an arbitrary point on the chart. &point is a pointer to an :cpp:struct:lv_point_t variable. E.g. lv_point_t point = {10, 20}. If the chart is scrolled, the cursor moves with it.

:cpp:expr:lv_chart_get_point_pos_by_id(chart, series, id, &point_out) gets the coordinate of a given point on the chart. This is useful to place the cursor on that data point.

:cpp:expr:lv_chart_set_cursor_point(chart, cursor, series, point_id) places the cursor on the specified data point on the chart. If the point's position changes (via a new value or via scrolling), the cursor moves with the point. See an example of using this function here <#show-cursor-on-the-clicked-point>_.

.. _lv_chart_events:

Events

:cpp:enumerator:LV_EVENT_VALUE_CHANGED Sent when a new point on the chart is pressed. :cpp:expr:lv_chart_get_pressed_point(chart) returns the zero-based index of the pressed point.

.. admonition:: Further Reading

Learn more about :ref:`lv_obj_events` emitted by all Widgets.

Learn more about :ref:`events`.

.. _lv_chart_keys:

Keys

No Keys are processed by Chart Widgets.

.. admonition:: Further Reading

Learn more about :ref:`indev_keys`.

.. _lv_chart_example:

Examples

.. include:: /examples/widgets/chart/index.rst

.. _lv_chart_api:

API