I suggest that for testing purposes you turn off this custom code that sets the Margin of the track ball. Then when the track ball reaches the end of the plot area you will see that it is not centered in consideration to the track line, but instead it is moved fully inside the plot area. This is done so that the track ball does not get clipped and does not overflow the chart space.
You need to take this in mind when you build your Margin work-around. You can get where the track ball will be positioned from the Canvas.Left and Canvas.Top values from the TrackBallInfoControl.
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