Hello Paul,
Unfortunately the RadDiagram component supports only Silverlight 5 and WPF 4. That is by design and we don't have plans to support older versions for this component.
Do you have plans to migrate to Silverlight 5? We can assist you in case you decided to do so.
Please let us know if you have further questions.
All the best,
Miro Miroslavov
the Telerik team
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