Indicate a possible 'lmodern' requirement

Author: uihsnv

lib/matplotlib/backends/backend_pgf.py

@@ -777,6 +777,10 @@ def _print_pgf_to_fh(self, fh, *, bbox_inches_restore=None):
 %% Make sure the required packages are loaded in your preamble
 %%   \\usepackage{pgf}
 %%
+%% Also ensure that all the required font packages are loaded; for instance, 
+%% the lmodern package is often necessary when using math font.
+%%   \\usepackage{lmodern}
+%%
 %% Figures using additional raster images can only be included by \\input if
 %% they are in the same directory as the main LaTeX file. For loading figures
 %% from other directories you can use the `import` package