The reason for this issue is called Sampling. The Chart combines it's items to avoid unclear visual representation (2 or more items shown for a single pixel), so that they are limited to a certain number (200
by default). You can read more about Sampling here - http://www.telerik.com/help/silverlight/radchart-features-sampling.html
To be able to show "#DATAITEM.MyProperty...." in Tooltip for > 200
items you should disable Sampling by setting SamplingTreshold to 0.
the Telerik team
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