example(chart) add area chart example (#2507)

* feat add stacked area chart example

The example shows how to use a line chart and masking to create a stacked area chart.

* feat stacked area chart example

Updated index.rst to contain the stacked area chart example

* feat stacked area chart example

Coding style changes

* Update changelog

* feat(example) stacked area chart

Added axis ticks

* feat(example) stacked area chart

Responses to PR comments
- Changed to fixed point maths
- Renamed variables and filenames

* feat(example) stacked area chart

Made shift amount a variable.

* feat(example) stacked area chart

Fixed typo in changelog

Co-authored-by: Tom Hepworth <Tom_Hepworth@mentor.com>
Co-authored-by: Gabor Kiss-Vamosi <kisvegabor@gmail.com>

docs/CHANGELOG.md

@@ -1,10 +1,10 @@
 # Changelog
 
 ## v8.1.0 (In progress)
+- feat(example) stacked area chart lv_example_chart_8 added.
 - renamed lv_obj_get_child_id(obj) to lv_obj_get_index(obj).
 - added lv_obj_move_to_index(obj, index). (#2461)
 - redefined lv_obj_move_foreground(obj) and lv_obj_move_background(obj) as inline functions now calling lv_obj_move_to_index(obj, index).
-- lv_obj_swap(obj1, obj2) added. (#2461)
 - feat(anim) add interface for handling lv_anim user data. (#2415)
 - feat(obj) add lv_is_initialized (#2402)
 - feat(obj) Backport keypad and encoder scrolling from v7 `lv_page` to v8 `lv_obj` (#2390)
@@ -21,7 +21,6 @@
 - feat(timer) check if lv_tick_inc is called <a href="https://github.com/lvgl/lvgl/commit/aa6641a6">aa6641a6</a> 
 - feat(docs) add view on GitHub link <a href="https://github.com/lvgl/lvgl/commit/a716ac6e">a716ac6e</a> 
 - feat(event) pass the scroll aniamtion to LV_EVENT_SCROLL_BEGIN <a href="https://github.com/lvgl/lvgl/commit/ca54ecfe">ca54ecfe</a> 
-
 - perf(draw) reimplement rectangle drawing algorithms 
 - perf(draw) reimplement circle drawing algorithms (#2374) (Also [change masking](https://docs.lvgl.io/master/overview/drawing.html#masking))
 - fix(draw) false assertion error in lv_draw_mask caused by wrong pointer

examples/widgets/chart/index.rst

@@ -42,4 +42,8 @@ Scatter chart
 .. lv_example:: widgets/chart/lv_example_chart_7
   :language: c
 
+Stacked area chart
+"""""""""""""""""""""""""""""""""""
 
+.. lv_example:: widgets/chart/lv_example_chart_8
+  :language:

examples/widgets/chart/lv_example_chart_8.c

@@ -0,0 +1,130 @@
+#include "../../lv_examples.h"
+#if LV_USE_CHART && LV_DRAW_COMPLEX && LV_BUILD_EXAMPLES
+
+/*  A struct is used to keep track of the series list because later we need to draw to the series in the reverse order to which they were initialised. */
+typedef struct
+{
+    lv_obj_t *obj;
+    lv_chart_series_t *series_list[3];
+} stacked_area_chart_t;
+
+static stacked_area_chart_t stacked_area_chart;
+
+/**
+ * Callback which draws the blocks of colour under the lines
+ **/
+static void draw_event_cb(lv_event_t *e)
+{
+    lv_obj_t *obj = lv_event_get_target(e);
+
+    /*Add the faded area before the lines are drawn*/
+    lv_obj_draw_part_dsc_t *dsc = lv_event_get_draw_part_dsc(e);
+    if (dsc->part == LV_PART_ITEMS)
+    {
+        if (!dsc->p1 || !dsc->p2)
+            return;
+
+        /*Add a line mask that keeps the area below the line*/
+        lv_draw_mask_line_param_t line_mask_param;
+        lv_draw_mask_line_points_init(&line_mask_param, dsc->p1->x, dsc->p1->y, dsc->p2->x, dsc->p2->y, LV_DRAW_MASK_LINE_SIDE_BOTTOM);
+        int16_t line_mask_id = lv_draw_mask_add(&line_mask_param, NULL);
+
+        /*Draw a rectangle that will be affected by the mask*/
+        lv_draw_rect_dsc_t draw_rect_dsc;
+        lv_draw_rect_dsc_init(&draw_rect_dsc);
+        draw_rect_dsc.bg_opa = LV_OPA_COVER;
+        draw_rect_dsc.bg_color = dsc->line_dsc->color;
+
+        lv_area_t a;
+        a.x1 = dsc->p1->x;
+        a.x2 = dsc->p2->x;
+        a.y1 = LV_MIN(dsc->p1->y, dsc->p2->y);
+        a.y2 = obj->coords.y2 - 13; /* -13 cuts off where the rectangle draws over the chart margin. Without this an area of 0 doesnt look like 0 */
+        lv_draw_rect(&a, dsc->clip_area, &draw_rect_dsc);
+
+        /*Remove the mask*/
+        lv_draw_mask_free_param(&line_mask_param);
+        lv_draw_mask_remove_id(line_mask_id);
+    }
+}
+
+/**
+ * Helper function to round a fixed point number
+ **/
+static int32_t round_fixed_point(int32_t n, int8_t shift)
+{
+    /* Create a bitmask to isolates the decimal part of the fixed point number */
+    int32_t mask = 1;
+    for (int32_t bit_pos = 0; bit_pos < shift; bit_pos++)
+    {
+        mask = (mask << 1) + 1;
+    }
+
+    int32_t decimal_part = n & mask;
+
+    /* Get 0.5 as fixed point */
+    int32_t rounding_boundary = 1 << (shift - 1);
+
+    /* Return either the integer part of n or the integer part + 1 */
+    return (decimal_part < rounding_boundary) ? (n & ~mask) : ((n >> shift) + 1) << shift;
+}
+
+/**
+ * Stacked area chart
+ */
+void lv_example_chart_8(void)
+{
+    /*Create a stacked_area_chart.obj*/
+    stacked_area_chart.obj = lv_chart_create(lv_scr_act());
+    lv_obj_set_size(stacked_area_chart.obj, 200, 150);
+    lv_obj_center(stacked_area_chart.obj);
+    lv_chart_set_type(stacked_area_chart.obj, LV_CHART_TYPE_LINE);
+    lv_chart_set_div_line_count(stacked_area_chart.obj, 5, 7);
+    lv_obj_add_event_cb(stacked_area_chart.obj, draw_event_cb, LV_EVENT_DRAW_PART_BEGIN, NULL);
+
+    /* Set range to 0 to 100 for percentages. Draw ticks */
+    lv_chart_set_range(stacked_area_chart.obj,LV_CHART_AXIS_PRIMARY_Y,0,100);
+    lv_chart_set_axis_tick(stacked_area_chart.obj, LV_CHART_AXIS_PRIMARY_Y, 3, 0, 5, 1, true, 30);
+
+    /*Set point size to 0 so the lines are smooth */
+    lv_obj_set_style_size(stacked_area_chart.obj, 0, LV_PART_INDICATOR);
+
+    /*Add some data series*/
+    stacked_area_chart.series_list[0] = lv_chart_add_series(stacked_area_chart.obj, lv_palette_main(LV_PALETTE_RED), LV_CHART_AXIS_PRIMARY_Y);
+    stacked_area_chart.series_list[1] = lv_chart_add_series(stacked_area_chart.obj, lv_palette_main(LV_PALETTE_BLUE), LV_CHART_AXIS_PRIMARY_Y);
+    stacked_area_chart.series_list[2] = lv_chart_add_series(stacked_area_chart.obj, lv_palette_main(LV_PALETTE_GREEN), LV_CHART_AXIS_PRIMARY_Y);
+
+    for (int point = 0; point < 10; point++)
+    {
+        /* Make some random data */
+        uint32_t vals[3] = {lv_rand(10, 20), lv_rand(20, 30), lv_rand(20, 30)};
+
+        int8_t fixed_point_shift = 5;
+        uint32_t total = vals[0] + vals[1] + vals[2];
+        uint32_t draw_heights[3];
+        uint32_t int_sum = 0;
+        uint32_t decimal_sum = 0;
+
+        /* Fixed point cascade rounding ensures percentages add to 100 */
+        for (int32_t series_index = 0; series_index < 3; series_index++)
+        {
+            decimal_sum += (((vals[series_index] * 100) << fixed_point_shift) / total);
+            int_sum += (vals[series_index] * 100) / total;
+
+            int32_t modifier = (round_fixed_point(decimal_sum, fixed_point_shift) >> fixed_point_shift) - int_sum;
+
+            /*  The draw heights are equal to the percentage of the total each value is + the cumulative sum of the previous percentages.
+                The accumulation is how the values get "stacked" */
+            draw_heights[series_index] = int_sum + modifier;
+
+            /*  Draw to the series in the reverse order to which they were initialised.
+                Without this the higher values will draw on top of the lower ones.
+                This is because the Z-height of a series matches the order it was initialsied */
+            lv_chart_set_next_value(stacked_area_chart.obj, stacked_area_chart.series_list[3 - series_index - 1], draw_heights[series_index]);
+        }
+    }
+
+    lv_chart_refresh(stacked_area_chart.obj);
+}
+
+#endif

examples/widgets/lv_example_widgets.h

@@ -57,6 +57,7 @@ void lv_example_chart_4(void);
 void lv_example_chart_5(void);
 void lv_example_chart_6(void);
 void lv_example_chart_7(void);
+void lv_example_chart_8(void);
 
 void lv_example_checkbox_1(void);