The track ball uses the GetDataContext method of the chart which is internal and it uses the FindClosestPoints method which is private, so unfortunately there is no way to change the mechanism that does the searching.
I am a bit unsure about what you need to know about the zooming and finding points between x-min and x-max. The points are simply iterated and if they are within the range, they are being plotted. Here, again, the rendering is done by internal classes that cannot be overridden.
I am unsure about the 2nd part, but the first, the trackball - if you find our trackball to be slow, you can simply turn it off and implement a custom trackball. You can use your logic to detect the closest points and show a custom element on top of the chart. Let us know if you need more information.
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