PR summary

supersedes #31048
Add basic support for generic unicode OpenType mathematics fonts such as STIX Two Math, Cambria Math, DejaVu Math, etc.

Currently, mathematics text rendering through mathtext in matplotlib supports a hard-coded number of fonts (configured via mathtext.fontset). Its design presumably predates the specification of mathematics alphabets in the unicode standard. While it is possible to configure custom fonts (mathtext.fontset: custom), this requires to set separate fonts for upright, italic, fraktur, double-struck, etc. variants -- which is fundamentally incompatible with the way modern mathematics fonts are designed.

Unicode defines mathematical alphanumeric symbols as unique codepoints (see https://en.wikipedia.org/wiki/Mathematical_Alphanumeric_Symbols), in contrast to different fonts all defining different styles for the same ASCII characters/codepoints.
One relatively modern way to render mathematical formulas uses mathematics fonts such as STIX Two Math or Cambria Math, Asana Math, etc.. For LaTeX, this is implemented in the unicode-math package.
Instead of choosing a font based on the style (as it is currently done in matplotlib) to render the same codepoints, this maps alphanumeric characters to different codepoints based on the style, and render them from a single font.

Shortcomings of the status quo:

• It is not currently possible to use modern mathematics fonts when using mathtext for rendering of mathematical equations in matplotlib. Consequently, it is difficult to configure fonts that are visually consistent and may require the use of latex rendering.
• Font configuration may be perceived as somewhat complicated
• The mathtext codebase and font selection logic is relatively complex and inaccessible, with all the hardcoded fonts, fixes for their quirks, and fallbacks

Changes

This change implements basic functionality to use any installed unicode OpenType mathematics fonts for use in mathtext in a portable way. Currently, this can be enabled by setting the rcparams

mathtext.fontset: unicodemath
mathtext.mathfont: STIX Two Math # or any other font name that can be loaded by fontmanager

I could think of different ways to configure this, though.

Internally, I have implemented a separate class UnicodeMathFonts(TruetypeFonts) to no interfere with the existing fontsets.

Running the test currently requires STIX Two Math to be installed on the system. For that reason, I have added it to the test data. One may think about vendoring STIX Two Math or DejaVu Math via mpl-data instead.

Examples

import matplotlib.pyplot as plt
import matplotlib as mpl

mpl.rcParams['mathtext.fontset'] = 'unicodemath'

font_names = ['STIX Two Math', 'TeX Gyre Pagella Math', 'TeX Gyre Schola Math', 'Noto Sans Math', 'Latin Modern Math', 'TeX Gyre DejaVu Math']

for font in font_names:
    with mpl.rc_context({'mathtext.mathfont' : font}):
        fig = plt.figure(figsize=(3, 1.5), dpi=150)
        fig.suptitle(f'{font}')
        fig.text(.1, .2, r'$\frac{1}{2 \pi \mathrm{i}} \int_{\partial G} f(z) \mathrm{d}z = \sum_{\nu = 1}^n \left(\mathrm{res} f\right)(z_\nu)$')
        fig.text(.1, .45, r'$\bigcup_{i \in I} A_i := \left\{ x \mid \bigvee_i\left( i \in I \wedge x \in A_i\right)\right\}$')
        fig.text(.1, .7, r'$\mathbb{Q} = \left. \left(\mathbb{Z} \times \mathbb{Z} \setminus \{ 0\} \right) \right/ \sim $')
        fig.tight_layout()
        fig.savefig(f'{font}.png')

PR checklist

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