RadChart provides three EmptyPoint Behaviors out-of-the-box. I have attached screenshots of the three modes.
The closest functionality to what you require that is available out-of-the-box is Drop. If this is good enough for you, then you can enable it by setting *SeriesDefinition.EmptyPointBehavior
If you however require dotted line over empty points, then you use the workaround demonstrated in the attached project. It requires using a separate series for the dotted effect and a separate ItemsSource.
I hope this helps.
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