feat(example) chart example to add gap between the old and new data (#2565)

* fix(chart) draw line chart indicator (bullet)

There was a missing bullet if the previous point was LV_CHART_POINT_NONE

* feat(example) add chart example with gap in circular mode

* add missing prototype

examples/widgets/lv_example_widgets.h

@@ -58,6 +58,7 @@ 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_chart_9(void);
 
 void lv_example_checkbox_1(void);
 void lv_example_checkbox_2(void);