Silverlight does not support such type of dynamic resolution of XValuePath / YValuePath properties -- their values must be static. The framework does not support the DataTemplateSelector concept either but you can try to implement it on your own -- check the following article
to get you started.
As for your dictionary question -- the XValuePath must resolve to numeric (double) or DateTime property, string values (i.e. categorical mode) are not supported. So you will be able to bind to Dictionary<double, double> or Dictionary<DateTime, double> but not to Dictionary<string, double>.
All the best,
the Telerik team
Registration for Q1 2011 What’s New Webinar Week is now open. Mark your calendar for the week starting March 21st and book your seat
for a walk through all the exciting stuff we ship with the new release!