That's really strange. I couldn't bind SPRadChartWepPart
to a list in my subsite too but there was no exception. I just couldn't find my custom list amongst the ones in Manage Series Definitions section
After discussion with our developers I understood that the SPRadChartWebPart was created by design to be bound to lists from the site collection. Unfortunately it doesn't matter if your wep part is located in subsite or in the top-level site - your list won't be visible for it.
What I can suggest is that you consider moving your list in the site collection or creating your own web part with our Chart.
the Telerik team
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