Fix quiver key plot when angles='xy' and/or scale_units='xy'

[stevezhang1999]

PR summary

Closes #26316 #26748 #27008

Fix incorrect behavior for quiver key. When angles='xy' and/or scale_units='xy', incorrect results will be produced for polar and aitoff.

Problem:

This happens because ax.quiverkey() reuses quiver._make_verts() to produce the arrow. During the call, quiver._angles_lengths() will be called when angles='xy' and/or scale_units='xy'. This methods will calculate the results using [X, Y] input from quiver() anyway, therefore, causing the result quiverkey() be broadcasted to wrong demension (instead of 1). This also applies to Cartesian - even if the result looks correct, multiple arrows were plotted at the same place.

Fix:

1. ax.quiverkey() will call quiver._make_verts(..., angles) only with angles='uv'. This is because we are using degrees to describe the angle for quiver key, not two points.
2. quiver._angles_lengths() now explicitly requires inputing [X, Y], rather than using those from ax.quiver() call.
3. A new baseline image for test_quiver.test_quiver_key_xy - the old one looks different than how it is specified in the doc [Bug]: quiverkey behavior with quiver(scale_units="xy") #27008

PR checklist

• "closes #0000" is in the body of the PR description to link the related issue
• new and changed code is tested
• [NA] Plotting related features are demonstrated in an example
• [NA] New Features and API Changes are noted with a directive and release note
• Documentation complies with general and docstring guidelines

[dstansby]

This looks good to me, and I checked it fixed the linked issues 👍 Could you squash the commits together so the PR cleanliness test passes? I think there's some issue with commits/test images that squashing should fix.

[stevezhang1999]

This looks good to me, and I checked it fixed the linked issues 👍 Could you squash the commits together so the PR cleanliness test passes? I think there's some issue with commits/test images that squashing should fix.

Thanks for your kind comment and help! I missed this message as I have been away from mpl for a while. Please let me know if I can help.