Indeed, this seems to be a bug in the chart wizard. I have attached a small example of a similar chart. In order to get this running without sending database files I have used a DataTable to populate the chart.
As RadChart does not support DateTime values, but it supports their OLE Automation equivalents, so there are 3 double columns and not two double and one DateTime column. You can still use the same chart with a SqlDataSource -- just remove the entire code behind and update your SqlDataSource as shown in this forum post
, so that DateTime values are converted to doubles.
the Telerik team