You can create an event handler after drawing, where you can draw your material:
mychart.PostPaint += new EventHandler<ChartPaintEventArgs>(PostPaintEventHandler);
...
static void PostPaintEventHandler(object sender, ChartPaintEventArgs e)
{
e.ChartGraphics.DrawString(...);
}
ILSpy, dll MSChart.
Graphics.DrawString. , .
, .