When glyph is not found in cmr10, substitute from cmsy10.

Author: tfpf

lib/matplotlib/_mathtext.py

@@ -475,6 +475,14 @@ class UnicodeFonts(TruetypeFonts):
     symbol can not be found in the font.
     """
 
+    # Some glyphs are not present in the `cmr10` font, and must be brought in
+    # from `cmsy10`. Map the Unicode indices of those glyphs to the indices at
+    # which they are found in `cmsy10`.
+    _cmr10_substitutions = {
+        0x00D7: 0x00A3,  # Multiplication sign.
+        0x2212: 0x00A1,  # Minus sign.
+    }
+
     def __init__(self, *args, **kwargs):
         # This must come first so the backend's owner is set correctly
         fallback_rc = mpl.rcParams['mathtext.fallback']
@@ -541,11 +549,11 @@ def _get_glyph(self, fontname, font_class, sym):
             found_symbol = False
             font = self._get_font(new_fontname)
             if font is not None:
-                if font.family_name == "cmr10" and uniindex == 0x2212:
-                    # minus sign exists in cmsy10 (not cmr10)
+                if (uniindex in self._cmr10_substitutions
+                        and font.family_name == "cmr10"):
                     font = get_font(
                         cbook._get_data_path("fonts/ttf/cmsy10.ttf"))
-                    uniindex = 0xa1
+                    uniindex = self._cmr10_substitutions[uniindex]
                 glyphindex = font.get_char_index(uniindex)
                 if glyphindex != 0:
                     found_symbol = True