I suspect this is caused by rounding errors. Often when clicking on a point the point moves, no matter how steady the mouse is held. (The user wants to select a point to use Ctrl + Arrow keys)
I verified that the mouse position had in fact not changed, as I had already inherited from the tool.
Solved by checking if the mouse has moved in overridden EndDrag() and then calling CancelOperation if it hasn't.
Worth looking at to see if it can be fixed? 

Hi Kevin,

This issue is caused by the timer running in the background - it is used to allow auto increment of the axis when the data point is moved outside the axis bounds. We can correct this issue of course but that would lead to a different problem - you won't be able to move the point outside the current axis range...

Best Regards,
Nevron Support Team

So the timer running moves the clicked point. I don't understand.
On a related note, the expansion of the axis range is often out of all proportion to the initial range, and I have reported a bug separately about the axis expansion when all the data points in the only series are 0, so it might be worth a quick look?