When working with a huge number of data items (in the ItemsSource) the RadChart offers a feature called Sampling. To avoid unclear visual representation, and to improve performance, the chart combines several data items into a single data point.
For instance - if you have 1000 items in your items source - only 200 data points will be visualized (that is 200 bars, if you use bar series). That means that each data point contains information for 5 data items.
This is why the DataItem that is passed is null.
You can set how many data points to be visualized by setting the SamplingThreshold
property (by default it is 200).
In your case you might need to fully disable Sampling by setting SamplingThreshold
You can read about Sampling here
where you can also get information about Custom Sampling Functions which are very helpful to use in some scenarios where disabling Sampling will have a negative impact on the performance.
I hope this helps.
the Telerik team
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