Currently the ChartDataSource does not provide a way to get the data items that formed the data point (as a result of sampling) out-of-the-box. We are planning to introduce an API for that with our Q2 2013 release.
I have attached a sample project demonstrating how you can currently achieve this. The project is in Silverlight, however since the API of the control is exactly the same between WPF and Silverlight you can copy-paste it in your WPF project.
I hope this helps.
the Telerik team