Fix dvi baseline detector when \usepackage{chemformula} is used.

It's a bit a whack-a-mole, but heh...

Author: anntzer

lib/matplotlib/dviread.py

@@ -302,6 +302,7 @@ def _read(self):
         # Pages appear to start with the sequence
         #   bop (begin of page)
         #   xxx comment
+        #   <push, ..., pop>  # if using chemformula
         #   down
         #   push
         #     down
@@ -313,17 +314,24 @@ def _read(self):
         #         etc.
         # (dviasm is useful to explore this structure.)
         # Thus, we use the vertical position at the first time the stack depth
-        # reaches 3, while at least three "downs" have been executed, as the
+        # reaches 3, while at least three "downs" have been executed (excluding
+        # those popped out (corresponding to the chemformula preamble)), as the
         # baseline (the "down" count is necessary to handle xcolor).
-        downs = 0
+        down_stack = [0]
         self._baseline_v = None
         while True:
             byte = self.file.read(1)[0]
             self._dtable[byte](self, byte)
-            downs += self._dtable[byte].__name__ == "_down"
+            name = self._dtable[byte].__name__
+            if name == "_push":
+                down_stack.append(down_stack[-1])
+            elif name == "_pop":
+                down_stack.pop()
+            elif name == "_down":
+                down_stack[-1] += 1
             if (self._baseline_v is None
                     and len(getattr(self, "stack", [])) == 3
-                    and downs >= 4):
+                    and down_stack[-1] >= 4):
                 self._baseline_v = self.v
             if byte == 140:                         # end of page
                 return True

lib/matplotlib/tests/test_usetex.py

@@ -81,16 +81,18 @@ def test_minus_no_descent(fontsize):
     assert len({*heights.values()}) == 1
 
 
-@pytest.mark.skipif(not _has_tex_package('xcolor'),
-                    reason='xcolor is not available')
-def test_usetex_xcolor():
+@pytest.mark.parametrize('pkg', ['xcolor', 'chemformula'])
+def test_usetex_packages(pkg):
+    if not _has_tex_package(pkg):
+        pytest.skip(f'{pkg} is not available')
     mpl.rcParams['text.usetex'] = True
 
     fig = plt.figure()
     text = fig.text(0.5, 0.5, "Some text 0123456789")
     fig.canvas.draw()
 
-    mpl.rcParams['text.latex.preamble'] = r'\usepackage[dvipsnames]{xcolor}'
+    mpl.rcParams['text.latex.preamble'] = (
+        r'\PassOptionsToPackage{dvipsnames}{xcolor}\usepackage{%s}' % pkg)
     fig = plt.figure()
     text2 = fig.text(0.5, 0.5, "Some text 0123456789")
     fig.canvas.draw()