# Bold font weight for LaTeX axes label in matplotlib

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### Question :

Bold font weight for LaTeX axes label in matplotlib

In matplotlib you can make the text of an axis label bold by

plt.xlabel('foo',fontweight='bold')


You can also use LaTeX with the right backend

plt.xlabel(r'$phi$')


When you combine them however, the math text is not bold anymore

plt.xlabel(r'$phi$',fontweight='bold')


Nor do the following LaTeX commands seem to have any effect

plt.xlabel(r'$bf phi$')
plt.xlabel(r'$mathbf{phi}$')


How can I make a bold $phi$ in my axis label?

Unfortunately you can’t bold symbols using the bold font, see this question on tex.stackexchange.

As the answer suggests, you could use boldsymbol to bold phi:

r'$boldsymbol{phi}$'


You’ll need to load amsmath into the TeX preamble:

matplotlib.rc('text', usetex=True)
matplotlib.rcParams['text.latex.preamble']=[r"usepackage{amsmath}"]


If you intend to have consistently bolded fonts throughout the plot, the best way may be to enable latex and add boldmath to your preamble:

# Optionally set font to Computer Modern to avoid common missing font errors
matplotlib.rc('font', family='serif', serif='cm10')

matplotlib.rc('text', usetex=True)
matplotlib.rcParams['text.latex.preamble'] = [r'boldmath']


Then your axis or figure labels can have any mathematical latex expression and still be bold:

plt.xlabel(r'$frac{phi + x}{2}$')


However, for portions of labels that are not mathematical, you’ll need to explicitly set them as bold:

plt.ylabel(r'textbf{Counts of} $lambda$'}


ax.set_ylabel(r"$mathbf{partial y / partial x}$")

As this answer Latex on python: alpha and beta don’t work? points out. You may have a problem with b so boldsymbol may not work as anticipated. In that case you may use something like: '$\boldsymbol{\beta}$' in your python code. Provided you use the preamble plt.rcParams['text.latex.preamble']=[r"usepackage{amsmath}"]