BUG: restore yt's classic math font even with matplotlib >= 3.4

tests/matplotlibrc

@@ -1,4 +1,3 @@
 #### MATPLOTLIBRC FORMAT
 
 backend : Agg
-mathtext.fontset : cm

yt/visualization/plot_container.py

@@ -2,12 +2,13 @@
 import builtins
 import os
 import textwrap
-from collections import defaultdict
+from collections import ChainMap, defaultdict
 from functools import wraps
 
 import matplotlib
 import numpy as np
 from matplotlib.cm import get_cmap
+from matplotlib.font_manager import FontProperties
 from more_itertools.more import always_iterable
 
 from yt._maintenance.deprecation import issue_deprecation_warning
@@ -236,6 +237,16 @@ def __init__(self, data_source, default_factory=None):
  return defaultdict.__init__(self, default_factory)
 
 
+_DEFAULT_FONT_PROPERTIES = {
+ "family": "stixgeneral",
+ "math_fontfamily": "cm",
+ "size": 18,
+ "fname": os.path.join(
+ matplotlib.get_data_path(), "fonts", "ttf", "STIXGeneral.ttf"
+ ),
+}
+
+
 class PlotContainer:
  """A container for generic plots"""
 
@@ -248,19 +259,17 @@ class PlotContainer:
  _units_config: dict
 
  def __init__(self, data_source, figure_size, fontsize):
- from matplotlib.font_manager import FontProperties
-
  self.data_source = data_source
  self.ds = data_source.ds
  self.ts = self._initialize_dataset(self.ds)
  if is_sequence(figure_size):
  self.figure_size = float(figure_size[0]), float(figure_size[1])
  else:
  self.figure_size = float(figure_size)
- font_path = os.path.join(
-  matplotlib.get_data_path(), "fonts", "ttf", "STIXGeneral.ttf"
- )
- self._font_properties = FontProperties(size=fontsize, fname=font_path)
+
+ font_props = _DEFAULT_FONT_PROPERTIES.copy()
+ font_props["size"] = fontsize
+ self._font_properties = FontProperties(**font_props)
  self._font_color = None
  self._xlabel = None
  self._ylabel = None
@@ -508,17 +517,15 @@ def set_font(self, font_dict=None):
  ... )
 
  """
- from matplotlib.font_manager import FontProperties
 
  if font_dict is None:
  font_dict = {}
  if "color" in font_dict:
  self._font_color = font_dict.pop("color")
  # Set default values if the user does not explicitly set them.
  # this prevents reverting to the matplotlib defaults.
- font_dict.setdefault("family", "stixgeneral")
- font_dict.setdefault("size", 18)
- self._font_properties = FontProperties(**font_dict)
+ font_props = ChainMap(font_dict, _DEFAULT_FONT_PROPERTIES)
+ self._font_properties = FontProperties(**font_props)
  return self
 
  def set_font_size(self, size):