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That's what it was! My mouse was hovering over the tooltip, and then hitting the left bracket would cause that option to be selected! If I move the mouse away it behaves as expected. Many thanks for your help.
That's exactly what I was after Russell, thanks! I just needed to set the x and y axis to the same scale, and the PlotRange did just that. I looked at that before but the documentation wasn't clear on which part was the x or y coordinates. Much...