The simplest way to draw a polygon on the chart is to use Annotations. These support transparency, they scale correctly when zooming and panning. If you want to add a Custom Annotation to a chart, please see this documentation here:
Please let me know if this helps,
The problem is that if I use an annotation the coordinates are different from the one of the serie, on top of that, I need to place the annotation in such a way that it aligns exactly over the serie. It would be much better if I could draw directly on the chart surface as the normal series do.
Annotation Coordinates should be exactly the same as the DataSeries coordinates so long as the annotation is on the same Axis as the series.
However, if your preference is to draw using the same API as RenderableSeries then please see our documentation article The Immediate Mode RenderContext API.
This supports FillPolygon and you can draw polygons outlines with DrawLine.
Have a look at our CustomRenderableSeries API. It is possible to override one of our series, or create your own, to draw polygons on top of, or under chart series.
Note that when using the RenderContext, all drawing is done in screen coordinates. It is possible to convert form Data to Screen coordinates (so that polygons scale with series) by following the examples in the CustomRenderableSeries API page.
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