Fix the vertical alignment of overunder symbols.

[tfpf]

This fixes #18085.

When an overunder symbol has a subscript, it goes below the symbol, and the baseline of the Vlist thus created is actually the baseline of the lowest item in the Vlist. Hence, it must be moved only if there is a subscript, and not otherwise.

PR Summary

MWE derived from the above-mentioned issue.

import matplotlib.pyplot as plt
(fig, ax) = plt.subplots()
ax.text(.01, .7, r"$\sum x\quad\sum^nx\quad\sum_nx\quad\sum_n^nx$", size=35); ax.axhline(.7)
ax.text(.01, .3, r"$\prod x\quad\prod^nx\quad\prod_nx\quad\prod_n^nx$", size=35); ax.axhline(.3)
plt.show()

Before:

After:

(Admittedly, the spacing could be much better. It is just right if there is no subscript and superscript, but not otherwise.)

PR Checklist

Tests and Styling

• Has pytest style unit tests (and pytest passes).
• Is Flake 8 compliant (install flake8-docstrings and run flake8 --docstring-convention=all).

Documentation

• [N/A] New features are documented, with examples if plot related.
• [N/A] New features have an entry in doc/users/next_whats_new/ (follow instructions in README.rst there).
• [N/A] API changes documented in doc/api/next_api_changes/ (follow instructions in README.rst there).
• [N/A] Documentation is sphinx and numpydoc compliant (the docs should build without error).

[oscargus]

It would be nice with some tests (primarily so that this isn't broken later), but I cannot really figure out a way to do that without adding yet another set of images (which should be avoided to reduce the git load). I think I'll open an issue an try to collect things to put in new tests.

[jklymak]

I don't think we should be avoiding image tests where they are needed, like this PR.

[oscargus]

I was quite sure that I recently saw a PR going in which some tests were removed, so I was about to give you some input on what else to add in the test, but I cannot seem to find it now...

I must admit that I do not really understand when test images should be added and not.

[jklymak]

There are lots of things that can be tested with "figures_equal" or with hard coded value tests. Other things, however, really require a render to test. In this case I don't think there is anyway to test that the rendering is correct without an image test.

[tfpf]

All right, I'll add a test here with \sum and \prod, perhaps similar to the MWE above. (Not very familiar with adding image comparison tests, but I can try.)

Should the test go in math_tests or lightweight_math_tests?

[timhoffm]

Not an expert here, but I think there should be no differences between fonts, so lightweight_math_tests is sufficient.

I hope this helps with writing the image comparison test:
https://matplotlib.org/stable/devel/testing.html#writing-an-image-comparison-test

[tfpf]

I added the test. Is there anything else to be done?

[oscargus]

should be no differences between fonts

You are correct in that it should not be any difference, but I think there is (at least based on other examples). However, I still think this is enough as things like spacing is not really what it could be. The PR solves the problem it claims to do, so I think it is good to go.

[timhoffm]

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[tfpf]

Thanks! Yep, will most certainly hang around!