The question is: how to enable panning/zooming operation to be constrained to the x axis or y axis?

The Tutorial says,
[about Pam Mode] If you press 'x' or 'y' while panning, the motion will be contrained to the x or y axis, respectively
[about Zoom Mode] You can use the modifier keys 'x', 'y' or 'CONTROL' to constrain the zoom to the x axes, the y axes, or aspect ratio preserve, respectively.

I find it really works for some examples, such as stock_demo.py.
However, it currently can't work on my canvas.
Is there any special setting so as to enable the functionality?


Thanks for your guidance in advance.

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sunzen
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