Our local tests do not show any issues related to RadAreaSparkline and inheriting from it seems to work correctly on our end. There is a difference, however, between RadGridView and RadSparkLine. While RadGridView just displays the data, RadAreaSparkline needs to be configured to interpret that data in order to transfrom it to a graphical view. Besides providing ItemsSource, you need to set the XValuePath and YValuePath properties as described in this help topic
So, you can still use type independent ItemsSource (it just has to be IEnumerable) but you need to perfrom the extra step of teaching the control how to read that data using the properties mentioned above.
the Telerik team
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