Thank you for contacting us.
This issue seems like a bug indeed. I have forwarded it to our developers so they will take a look at it. Please note that we have newer version of the RadChart control - RadChartView, and we focused our efforts on it, so I cannot commit with a specific timeframe and whether or when this issue would be fixed.
I would suggest you to take a look at the RadChartView, where the same scenario is working correct.
More about the RadChartView you can read here
and also can see our online demos
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