One possible option in your case would be to use marked zones. This is demonstrated in the code snippet below:
void RadChart1_BeforeLayout(object sender, EventArgs e)
ChartMarkedZone item = new ChartMarkedZone("zone1");
item.ValueStartY = 2;
item.ValueEndY = 4;
I hope that this is a feasible option for you.
the Telerik team
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