Problem

Now that #30405 is merged, we can think about extending the functionality for single-axis zoom.

Proposed solution

Possible approaches:

• Use additional modifiers: I don't think there are standard conventions as most applications have fixes aspects and don't need to zoom in different axis. My proposal would be

  • Ctrl+Wheel zoom both axis
  • Shift+Wheel zoom y
  • Alt+Wheel zoom x
    Rationale: adding additional modifiers Ctrl+Shift+Wheel is cumbersome to use and does not add another benefit, so just replace the modifier. Mnemonic from the keyboard layout

  <Shift>
  <Ctrl>   <Alt>
  

  so Shift is on the vertical Alt is on the horizontal.

• Use special regions: Limit scroll to one direction for Ctrl+Wheel if the cursor is over an axis. This would be quite neat, but I think we do not have a good "over an axis definition. Picking the spine is too fiddly. We'd need area outside the axis includding the ticks. Something like this. But I think we have no efficent way to determine that.
  

Feedback welcome.