add highlight circle for radar chart

[liuxuan30]

For #138
Add highlight circle for radar chart. It combined an inner filled circle and an outer stroke circle. inner circle fill color, inner circle radius, outer circle radius, stroke width and alpha can be customised.

I notice drawHighlightLines is in LineScatterCandleRadarChartRenderer now, no longer in the radar chart renderer. I am not sure if draw highlight circle should be here.

Since there are several properties can be asked to customized, I create new properties for radar chart view.

Let me know if we want to add more customizable properties or it's too many, as well as if the structure needs to be improved.

[preetirani]

Hey @liuxuan30
i want to draw circles in radar chart for the selected dataset. Can you please help me to understand what all properties needs to b implemented on radarchatview.
Here are the properties I have set but circles are not showing at all.
set1.drawHighlightCircleEnabled = true
set1.highlightCircleStrokeAlpha = 1
set1.highlightCircleFillColor = UIColor.green
set1.highlightCircleOuterRadius = 10
set1.highlightCircleInnerRadius = 8
set1.highlightCircleStrokeColor = UIColor.white