[graph] zoom-in and zoom-out misaligned

[saulpw]

Discussed in #2106

Zooming in and then back out of a graph should take you back to your original starting point. Now, it goes slightly off until eventually the plot disappears off the screen.

Expected Behavior

This is the expected behavior when you zoom-in and out on a graph with your mouse-wheel:

And this is the develop behavior:

[saulpw]

@isosphere This was working fine before with geometric zoom. I tried linear zoom and it didn't 'feel' right. This isn't a precision issue; likely just an off-by-one error from a change we made to the canvas in ~august or so.

[midichef]

@anjakefala
I am leaving a comment here so you can assign this to me, as you asked in the Discussion related to this.

I've figured out what's happening here. Graph andInvertedCanvas use a different coordinate system from Canvas. Their y-axis grows in the opposite direction and starts from a different origin point. zoomin/out-mouse incorrectly rely on Canvas.fixPoint(), which uses the y-coordinate system for Canvas. InvertedCanvas needs its own version of fixPoint(). I'm testing a fix now.

If there are more such bugs in graph.py discovered in the future, the fix would follow this general rule: look for functions being called in graph.py that are in Canvas that involve y-coordinate calculations. Override them in InvertedCanvas and modify them to use inverted y-coordinates.

[anjakefala]

If there are more such bugs in graph.py discovered in the future, the fix would follow this general rule: look for functions being called in graph.py that are in Canvas that involve y-coordinate calculations. Override them in InvertedCanvas and modify them to use inverted y-coordinates.

This was such a helpful write-up, thank you!