The tooltips in the ASP RadChart are produced using Image Maps. They display the string in the radChart.PlotArea.XAxis.Items[index].ActiveRegion.Tooltip property. Unfortunately binding them is not supported and the only way to set their value is to manually traverse the radChart.PlotArea.XAxis.Items collection on the DataBound event and set each of the tooltip of the X Axis labels your own value.
I hope this helps.
the Telerik team
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