Currently, RadChart for WP7 treats null values like 0, that is, it does not have explicit support for null values. For example if you have line series in the chart, you won't have discontinuities in the line, instead, for every null value the line will return to 0.
This is a highly requested feature and is planned for the Q2 2012 release.
Thank you for understanding.
the Telerik team
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