The issue you're experiencing is caused by setting the IntelligentLabelsEnabled property of RadChart to true. This feature has been added to prevent item labels from overlapping each other when there are too many items in a series, just as you've guessed.
Therefore, we can suggest 2 courses of action: you can either set IntelligentLabelsEnabled to false or you can give the RadChart more space to render itself. Please, consider these and see how they'll work out for you.
the Telerik team
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