You can achieve your requirement by using the Position property of the TrackBallBehavior. Basically you can subscribe for the MouseLeftButtonDown event of the chart and inside its handler get the current mouse position and set it as the trackball's Position. However, this won't prevent the moving of the trackball on mouse move. To resolve this you can save the previously clicked position and set it in the PositionChanging event handler of the behavior. Here is an example in code:
sender, TrackBallPositionChangingEventArgs e)
e.NewPosition = currentPosition;
sender, MouseButtonEventArgs e)
currentPosition = e.GetPosition(
.trackBallBehavior.Position = currentPosition;
Please give this approach a try and let me know if it works for you.