PGF: Set up font sizes to match font.size setting

This allows the output .pgf file to be used in external documents
with arbitrary font size settings (which should match the rc setting
font.size). This avoids mismatches between matplotlib and external
LaTeX documents for things that depend on the font size setting (e.g.
unicode-math).

If the LaTeX package scrextend is present, this also adjusts the
standard LaTeX font commands (\tiny, ..., \normalsize, ..., \Huge)
relative to font.size.

Author: Socob

lib/matplotlib/backends/backend_pgf.py

@@ -23,6 +23,7 @@
     _create_pdf_info_dict, _datetime_to_pdf)
 from matplotlib.path import Path
 from matplotlib.figure import Figure
+from matplotlib.font_manager import FontProperties
 from matplotlib._pylab_helpers import Gcf
 
 _log = logging.getLogger(__name__)
@@ -36,12 +37,27 @@
 
 def _get_preamble():
     """Prepare a LaTeX preamble based on the rcParams configuration."""
+    font_size_pt = FontProperties(
+        size=mpl.rcParams["font.size"]
+    ).get_size_in_points()
     return "\n".join([
         # Remove Matplotlib's custom command \mathdefault.  (Not using
         # \mathnormal instead since this looks odd with Computer Modern.)
         r"\def\mathdefault#1{#1}",
         # Use displaystyle for all math.
         r"\everymath=\expandafter{\the\everymath\displaystyle}",
+        # Set up font sizes to match font.size setting.
+        r"\makeatletter",
+        r"\IfFileExists{scrextend.sty}{",
+        r"  %s" % mpl.texmanager._usepackage_if_not_loaded(
+            "scrextend", option="fontsize=%fpt" % font_size_pt
+        ),
+        r"}{",
+        r"  \renewcommand{\normalsize}{\fontsize{%f}{%f}\selectfont}"
+        % (font_size_pt, 1.2 * font_size_pt),
+        r"  \normalsize",
+        r"}",
+        r"\makeatother",
         # Allow pgf.preamble to override the above definitions.
         mpl.rcParams["pgf.preamble"],
         *([