How to de-gradient bubbles in a bubble chart

I found the answer on my own. I solved it by using PaletteProvider and IPointMarkerPaletteProvider.

public class CustomPaletteProvider : IPointMarkerPaletteProvider
{
private XyzDataSeries<DateTime, double, double> _theSourceData;

public void OnBeginSeriesDraw(IRenderableSeries series)
{
_theSourceData = (XyzDataSeries<DateTime, double, double>)series.DataSeries;
}

public PointPaletteInfo? OverridePointMarker(
IRenderableSeries rSeries,
int index,
IPointMetadata metadata)
{
PointPaletteInfo info = new PointPaletteInfo();
info.Stroke = Colors.Gray;

return info;
}
}