lvgl/docs/widgets/chart.md

.. include:: /header.rst 
:github_url: |github_link_base|/widgets/chart.md

Chart (lv_chart)

Overview

Charts are a basic object to visualize data points. They support Line charts (connect points with lines and/or draw points on them) and Column charts.

Charts also support division lines, 2 y axis, axis ticks, and texts on ticks.

Parts and Styles

The Chart's main part is called LV_CHART_PART_BG and it uses all the typical background properties. The text style properties determine the style of the axis texts and the line properties determine ticks' style. Padding values add some space on the sides thus it makes the series area smaller. Padding also can be used to make space for axis texts and ticks.

The background of the series is called LV_CHART_PART_SERIES_BG and it's placed on the main background. The division lines, and series data is drawn on this part. Besides the typical background style properties the line style properties are used by the division lines. The padding values tells the space between the this part and the axis texts.

The style of the series can be referenced by LV_CHART_PART_SERIES. In case of column type the following properties are used:

• radius: radius of the bars
• padding_inner: space between the columns of the same x coordinate

In case of Line type these properties are used:

• line properties to describe the lines
• size radius of the points
• bg_opa: the overall opacity of the area below the lines
• bg_main_stop: % of bg_opa at the top to create an alpha fade (0: transparent at the top, 255: bg_opa at the top)
• bg_grad_stop: % of bg_opa at the bottom to create an alpha fade (0: transparent at the bottom, 255: bg_opa at the top)
• bg_drag_dir: should be LV_GRAD_DIR_VER to allow alpha fading with bg_main_stop and bg_grad_stop

LV_CHART_PART_CURSOR refres to the cursors. Any number of cursor can be added and their appearence can be set by the line related style properties. The color of the cursors are set when the cursor is created and line_color fro mteh style is overwritten by this value.

Usage

Data series

You can add any number of series to the charts by lv_chart_add_series(chart, color). It allocates data for a lv_chart_series_t structure which contains the chosen color and an array for the data points if not using an external array, if an external array is assigned any internal points associated with the series are deallocated and the series points to the external array instead.

Series' type

The following data display types exist:

• LV_CHART_TYPE_NONE - Do not display any data. It can be used to hide the series.
• LV_CHART_TYPE_LINE - Draw lines between the points.
• LV_CHART_TYPE_COLUMN - Draw columns.

You can specify the display type with lv_chart_set_type(chart, LV_CHART_TYPE_...). The types can be 'OR'ed (like LV_CHART_TYPE_LINE).

Modify the data

You have several options to set the data of series:

1. Set the values manually in the array like ser1->points[3] = 7 and refresh the chart with lv_chart_refresh(chart).
2. Use lv_chart_set_point_id(chart, ser, value, id) where id is the index of the point you wish to update.
3. Use the lv_chart_set_next(chart, ser, value).
4. Initialize all points to a given value with: lv_chart_init_points(chart, ser, value).
5. Set all points from an array with: lv_chart_set_points(chart, ser, value_array).

Use LV_CHART_POINT_DEF as value to make the library skip drawing that point, column, or line segment.

Override default start point for series

If you wish 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 lv_chart_set_x_start_point(chart, ser, id) where id is the new index position to start plotting from.

Set an external data source

You can make the chart series update from an external data source by assigning it with the function: lv_chart_set_ext_array(chart, ser, array, point_cnt ) where array is an external array of lv_coord_t with point_cnt elements. Note: you should call lv_chart_refresh(chart) after the external data source has been updated, to update the chart.

Get current chart information

There are four functions to get information about a chart:

1. lv_chart_get_type(chart) returns the current chart type.
2. lv_chart_get_point_count(chart) returns the current chart point count.
3. lv_chart_get_x_start_point(ser) returns the current plotting index for the specified series.
4. lv_chart_get_point_id(chart, ser, id) returns the value of the data at a particular index(id) for the specified series.

Update modes

lv_chart_set_next can behave in two ways depending on update mode:

• LV_CHART_UPDATE_MODE_SHIFT - Shift old data to the left and add the new one o the right.
• LV_CHART_UPDATE_MODE_CIRCULAR - Circularly add the new data (Like an ECG diagram).

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

Number of points

The number of points in the series can be modified by lv_chart_set_point_count(chart, point_num). The default value is 10. Note: this also affects the number of points processed when an external buffer is assigned to a series.

Vertical range

You can specify the minimum and maximum values in y-direction with lv_chart_set_range(chart, y_min, y_max). 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 lv_chart_set_div_line_count(chart, hdiv_num, vdiv_num). The default settings are 3 horizontal and 5 vertical division lines.

Tick marks and labels

Ticks and labels can be added to the axis.

lv_chart_set_x_tick_text(chart, list_of_values, num_tick_marks, LV_CHART_AXIS_...) set the ticks and texts on x axis. list_of_values is a string with '\n' terminated text (expect the last) with text for the ticks. E.g. const char * list_of_values = "first\nsec\nthird". list_of_values can be NULL. If list_of_values is set then num_tick_marks tells the number of ticks between two labels. If list_of_values is NULL then it specifies the total number of ticks.

Major tick lines are drawn where text is placed, and minor tick lines are drawn elsewhere. lv_chart_set_x_tick_length(chart, major_tick_len, minor_tick_len) sets the length of tick lines on the x-axis.

The same functions exists for the y axis too: lv_chart_set_y_tick_text and lv_chart_set_y_tick_length.

Cursor

A cursor can be added with lv_chart_cursor_t * c1 = lv_chart_add_cursor(chart, color, dir);. The possible values of dir LV_CHART_CURSOR_NONE/RIGHT/UP/LEFT/DOWN or their OR-ed values to tell in which direction(s) should the cursor be drawn.

lv_chart_set_cursor_point(chart, cursor, &point) sets the position of the cursor. point is a pointer to an lv_poin_t variable. E.g. lv_point_t point = {10, 20};. The point is relative to the series area of the chart.

The lv_coord_t p_index = lv_chart_get_nearest_index_from_coord(chart, x) tells which point index is to the closest to a X coordinate (relative to the series area). It can be used to snap the cursor to a point for example when the chart is clicked.

lv_chart_get_x_from_index(chart, series, id) and lv_chart_get_y_from_index(chart, series, id) tells the X and Y coordinate of a given point. It's useful to place the cursor to given point.

The current series area can be retrieved with lv_chart_get_series_area(chart, &area) where area is a pointer to an lv_area_t variable to store the result. The area has absolute coordinates.

Events

Only the Generic events are sent by the object type.

Learn more about Events.

Keys

No Keys are processed by the object type.

Learn more about Keys.

Example

.. include:: /lv_examples/src/lv_ex_widgets/lv_ex_chart/index.rst

API

.. doxygenfile:: lv_chart.h
  :project: lvgl