I note that the existing x-/y- modifiers do not really work when using my laptop's touchpad¹, but this proposal does work nicely 👍

This is an interesting concept, though I was a bit surprised at first. Is there any precedence that other programs use this approach?

Also, would it be a good idea to give some sort of visual aide, with the goals (i) to make it clear that something special is going on and (ii) give an indication of the area the mouse has to be in so that the special handling applies? e.g. something like marking the region:

Matlab has similar behavior. Could implement something similar where the lines at the edge indicate the region the mouse needs to be in?

The range marker is a good visualization to indicate the difference. Though, I would like to additionally keep dashed lines as an extension of the whiskers, because that helps to ensure relevant data points are included.

That said, all the visual changes would need refactoring and backend-dependent implementation as currently all visualization is done via RubberBandBase and that does only cover the rectangle without the ability for further modification.

Example of whiskers and dashed rectangle using QtCairo and QtAgg backends. The width of the whiskers represents the region the mouse needs to be in for single-axis zoom.

Excuse the lack of mouse pointer in the screenshot.