At runtime, RadChart is just an image that doesn't offer interactivity beyond the basics provided by a standard image in .NET Framework. Therefore, manipulating individual items inside RadChart is not supported.
In addition, for future projects you should consider using the new RadChartView for WinForms instead of RadChart where applicable. RadChartView supercedes the now obsolete RadChart. For a more detailed comparison between the two, please check out this help topic
the Telerik team