It seems there is a problem with the ChartSeries in this case. Please, clear the RadChart.Series
collection and re-create the series when changing the value in the ComboBox i.e. add these lines before assigning the chart datasource:
As a side note -- I have edited your message, so that the link is not displayed. Please, do not upload Telerik assemblies to public servers. You can still upload the project -- just remove the assembly from the archive.
the Telerik team