I have created a simple bar chart populated by observable items. I added a button to the app, so that when I click the button - an item increases its value. I do not get any flickering and the bar changes its size accordingly. I have attached this app for your reference.
If you keep experiencing issues with the RadChart, you can open a support ticket and attach a small project that we can run that reproduces this behavior so that we can try to find a way to avoid this flicker. For now I can only suggest you make sure the underlying items implement the INotifyPropertyChanged interface and that the collection they reside in implement the INotifyCollectionChaged interface.
All the best,
the Telerik team