Apply suggestion from @timhoffm, update lib/matplotlib/markers.py

Co-authored-by: Tim Hoffmann <2836374+timhoffm@users.noreply.github.com>

Mac qt ctrl

Fix clim handling for pcolor{,mesh}.

Fix format_cursor_data with nans.

Fix DATA_PARAMETER_PLACEHOLDER interpolation for quiver&contour{,f}.

... and small local cleanups.

- Use finite differences to estimate derivative rather than splines
  (which are conceptually much more complicated).
- Include a "constant error" case, which shows that the band does have
  constant width in that case.
- Force the axes aspect to 1 (so that constant width actually appears
  constant).
- Use np.full() to set up the codes array.

The pixel-sized marker previously use (",") was so small that it looked
more like a rendering artefact than an actual data point.

The error messages now provide better instruction on how to fix the
problem.

There are some paths we can not split (one with hatching, fills, or
that claim they can not be simplified) so tell user that.

If we could have otherwise split up the path if the chunksize is set
to less than 100, which disables the feature, ask user to make it
bigger or make the path simplification more aggressive.

If the chunksize is bigger than the data, ask user to make it smaller
or make the path simplification more aggressive.

If the chunksize is smaller than the data, but still too big ask user
to make it smaller or make the path simplification more aggressive.

closes matplotlib#19325

Co-authored-by: Antony Lee <anntzer.lee@gmail.com>

This allows longer, more textured, paths to be drawn with the default
settings at the risk of very long run times.

- Make sure values are be big enough / small enough
- make long path test more pathological (Random data will sometimes go in the
  same direction)

This prevents them accidentally breaking the test runner itself,
depending on platform.

DOC: Increase marker size in double_pendulum example.

DOC: Improve curve_error_band example.

DOC: Docstring cleanups around DATA_PARAMETER_PLACEHOLDER.

MNT: Simplify argument parsing in stem().

Co-authored-by: Tim Hoffmann <2836374+timhoffm@users.noreply.github.com>

This declutters the code.

Fix flake8 from matplotlib#21335

Simplify _init_legend_box.

ENH: Add support to save images in WebP format

Simplify wording of allowed errorbar() error values

Add GHA testing whether files were added and deleted in the same PR.

The exception message is clearer for end users than downstream callers
failing to `open()` a file named `""`.

Also update the function's docstring.

_tfmfile now never returns None (an exception would have been raised
earlier by find_tex_file), so remove the corresponding branch.

Raise an exception when find_tex_file fails to find a file.

Co-authored-by: Tim Hoffmann <2836374+timhoffm@users.noreply.github.com>

Describe possible need for loading the 'lmodern' package when using PGF files

Currently, the scatter_hist example needs to force the main axes to be
square by carefully adjusting the figure size -- shared axes and fixed
aspects don't work well together (and manually resizing the figure shows
that the aspect is indeed not fixed).  In fact, the
scatter_hist_locatable_axes example explicitly states that the advantage
of using axes_grid1 is to allow the aspect to be fixed.

I realized that one can also use inset_axes to position the marginals
axes relative to the main axes *and* support a fixed aspect ratio for
the main axes.  Perhaps this can be considered as a slight API abuse,
but I think this is a solution for a real limitation; the question is
whether we want to promote this use?

Demonstrate inset_axes in scatter_hist example.

We don't need to drop the alpha channel for jpeg anymore as imsave
handles that.

In imsave, also clarify that `sm` is not going to be used in the
already-usable-rgba case.

Also, spliting out pil_kwargs under "other parameters" seems overkill
for an effectively internal method (`savefig` is the user-facing API).

…tests

Enable tests for text path based markers

Document what indexing a GridSpec returns

Factor common parts of saving to different formats using pillow.

Log pixel coordinates in event_handling coords_demo example on terminal/console