$ is not for using italic, it is for render text as equations. There characters are italic and numbers not.

The seemingly correct way to do that is to instead write

plt.ylabel(r"$\mathit{CYP11B2}$ mRNA (%)")

However, if this works or not will depend on if the font you are using, determined by rcParams['mathtext.it'], has italic numbers or not.

It may be that there is a bug here, but I have not really figured out if it depends on the font or if, for some reasons, digits are not rendered using the italic font, as the default font is not rendered correctly.

Hello, can I work on this if this really is a bug?

plt.ylabel(r"$\mathit{CYP11B2}$ mRNA (%)")

I already tried this $\mathit{CYP11B2}$, it doesn't work. I'm currently using Arial and can't change the font type, since it is required by the journal. CYP11B2 needs to be fully italic, this is the biological nomenclature for gene names.

Why are you using mathtext?

Why are you using mathtext?

Is it possible to mix italic and upright fonts in a single label otherwise?

It does seem like we aren't consistent with Latex here. If I do

ax.set_ylabel(r'$\it{abc123}$', usetex=False)

only the letters are in italics, whereas if I do

ax.set_ylabel(r'$\it{abc123}$', usetex=True)

both letters and numbers are in italics.

@Sanas-m the first example above uses Matplotlib's mathtex whereas the second example (usetex=True) uses my locally installed Latex. This may be a workaround for you and you can set an rcParam to turn it on for all your text:
https://matplotlib.org/stable/users/explain/text/usetex.html

Why are you using mathtext?

Is it possible to mix italic and upright fonts in a single label otherwise?

Sorry, agree you need something tex-like if you want to mix italic and non-italics.

This looks like a bug to me (matching tex's behavior seems reasonable). The code that forces numbers to be in roman is under UnicodeFonts._get_glyph (commenting out the section starting at if fontname == 'it' and uniindex < 0x10000 revert them to italics), but doing that would turn all digits to italics in math mode (whereas we want r"1\mathit{1}" to only have the second one in italics). I suspect(?) the issue is that at the parsing stage all the characters are forced to italics (more precisely, to rcParams["mathtext.default"] by Parser.non_math), but really that should not affect digits (and greek capital letters); then this info can be threaded all the way down rather than doing the switch in UnicodeFonts._get_glyph.

It does seem like we aren't consistent with Latex here. If I do

ax.set_ylabel(r'$\it{abc123}$', usetex=False)

only the letters are in italics, whereas if I do

ax.set_ylabel(r'$\it{abc123}$', usetex=True)

both letters and numbers are in italics.

@Sanas-m the first example above uses Matplotlib's mathtex whereas the second example (usetex=True) uses my locally installed Latex. This may be a workaround for you and you can set an rcParam to turn it on for all your text: https://matplotlib.org/stable/users/explain/text/usetex.html

I tried this but then the fonts are mixed again