Improve error message for non-matching types with aspect set to equal

[hoxbro]

Fixes #6089

This can create the Matplotlib problem without rasterize. rasterize will have an Image aspect set to 'equal', whereas Quadmesh have an aspect set to square.

I have chosen to raise a TypeError instead of adding custom logic as it does not really make sense to take the equal of an axis with numbers and an axis with timedelta. An alternative could be to set the aspect to 1.

import holoviews as hv
import numpy as np
import pandas as pd

hv.extension("matplotlib")

X = pd.date_range(start="1/1/2018", end="1/08/2018", periods=100)
Y = np.linspace(1, 100, 100)
Z = np.random.randn(100, 100)

hv.QuadMesh((X, Y, Z)).opts(aspect='equal')

TODO

• Improve plotly
• Add tests

[codecov-commenter]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (f81533a) 88.66% compared to head (42a87f6) 88.66%.
Report is 1 commits behind head on main.

Files	Patch %	Lines
holoviews/plotting/mpl/element.py	33.33%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6103      +/-   ##
==========================================
- Coverage   88.66%   88.66%   -0.01%     
==========================================
  Files         314      314              
  Lines       65975    65985      +10     
==========================================
+ Hits        58497    58505       +8     
- Misses       7478     7480       +2     

Flag	Coverage Δ
ui-tests	23.67% <0.00%> (-0.01%)	⬇️

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