It seems that you are placing the GanttView over or inside the TimeBar control and this causes the observed effect. You could refer to this example
that demonstrates how to use the ScheduleView and TimeBar control - the GanttView is used almost the same way as the ScheduleView. The demo is for Silverlight, but exactly the same code could be used in WPF as well.
Hope this helps.
the Telerik team
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