Bug summary

I noticed that, when adding PGF pictures to a beamer document, not all the fonts in the picture are correct.
For example, in the slide

you can see that the slide title is using a certain font while the plot labels and legend are using a different font. Curiously, the units of measure, created with the siunitx package, do use the same font as the slide title, which is correct.

I inspected the generated pgf file and I noticed that all the labels are prefixed with \rmfamily. If I remove that command, then I get the correct output:

Also worth noting is that when I compile a regular document, with \documentclass{article} for example, I always get the right fonts for all labels in the plot, with or without \rmfamily.

I am not 100% sure why the \rmfamily instruction is there, but removing it fixed the output in beamer. Maybe it can be omitted? 🤔

Or maybe there's a valid LaTeX explanation for this?

Code for reproduction

# this code snippet is only a sketch of the actual one

import matplotlib
matplotlib.use("pgf")

import matplotlib.pyplot as plt

params = {
    'text.usetex'       : True,
    "pgf.texsystem": "pdflatex",
    'font.family': 'serif',
    'pgf.rcfonts': False,
    "pgf.preamble": r"\usepackage{siunitx}"
}

matplotlib.rcParams.update(params)

# omitted stuff...

plt.plot(x, d, "*", label="data")
plt.plot(x, y, label="linear regression")
plt.xlabel(r"input (\si{L/min})")
plt.ylabel(r"output  (\si{L/min})")
plt.legend()
plt.savefig("linearRegression.pgf")

Actual outcome

See the first figure in the description

Expected outcome

See the second figure in the description

Additional information

No response

Operating system

Linux KDE Neon

Matplotlib Version

3.7.1

Matplotlib Backend

No response

Python version

3.10.6

Jupyter version

No response

Installation

pip