How to use the \siunitx package within Python/matplotlib?

Question

I'm writing a LaTeX document and I include some matplotlib plots in it, in .pgf format with \include{file.pgf}. It has advantages over exporting the matplotlib plot to .pdf or .eps formats in that the fonts matches the one of my document and probably other advantages, but that is beside the main question.

In my LaTeX document I use the siunitx package, and I'd like to use it for the matplotlib plots too. For instance some axis might be the distance in meters, so I'd like to use Distance (\si{\meter}) instead of Distance (m).

From what I gathered from an extensive Google research, in the Python file .py, I came up with the following example:

import matplotlib as mpl
mpl.use('pgf')
mplparams = {"pgf.rcfonts": False}
mpl.rc('text', usetex=True)
mpl.rcParams.update(mplparams)
import matplotlib.pyplot as plt
plt.rc('text', usetex=True) 
if "siunitx" not in plt.rcParams["text.latex.preamble"]:
    plt.rcParams["text.latex.preamble"].append(r"\usepackage{siunitx}")

X = [0, 1, 2, 3, 4, 5]
Y = [-2, 7 , 3 , 8 , 7, 5]
plt.plot(X, Y)
plt.xlabel('Distance (\si{\meter})')
plt.savefig('file.pgf')

However it won't work and yields the error message:

  File "(...)\Anaconda3\lib\site-packages\matplotlib\backends\backend_pgf.py", line 375, in get_width_height_descent
    raise ValueError(msg % (text, e.latex_output))

ValueError: Error processing 'Distance (\si{\meter})'
LaTeX Output:
! Undefined control sequence.
<argument> ...12.000000}\selectfont Distance (\si 
                                              {\meter })
<*> ....000000}\selectfont Distance (\si{\meter})}

No pages of output.
Transcript written on texput.log.

If I replace \si{\meter} by m, it does work.

I am therefore unable to use the siunitx package within Python. How could I make it work?

Answer 1

This is probably not the correct place to ask this, as the question is about Matplotlib and Python not about LaTeX, IMHO. I use siunitx in the following way:

pgf_with_latex = {                      # setup matplotlib to use latex for output
    "pgf.texsystem": "pdflatex",        # change this if using xetex or lautex
    "text.usetex": True,                # use LaTeX to write all text
    "font.family": "serif",
    "font.serif": [],                   # blank entries should cause plots 
    "font.sans-serif": [],              # to inherit fonts from the document
    "font.monospace": [],
    "axes.labelsize": 10,               # LaTeX default is 10pt font.
    "font.size": 10,
    "legend.fontsize": 8,               # Make the legend/label fonts 
    "xtick.labelsize": 8,               # a little smaller
    "ytick.labelsize": 8,
    "figure.figsize": figsize(0.9),     # default fig size of 0.9 textwidth
    "pgf.preamble": "\n".join([ # plots will use this preamble
        r"\usepackage[utf8]{inputenc}",
        r"\usepackage[T1]{fontenc}",
        r"\usepackage[detect-all,locale=DE]{siunitx}",
        ])
    }
mpl.rcParams.update(pgf_with_latex)

So you'd have to update the pgf.preamble to include r"\usepackage{siunitx}".