Support minus signs for pgf backend in combination with pdflatex

Add test and fix for issue matplotlib#18826

Author: Philipp Arras

lib/matplotlib/backends/backend_pgf.py

@@ -100,6 +100,7 @@ def common_texification(text):
     # Sometimes, matplotlib adds the unknown command \mathdefault.
     # Not using \mathnormal instead since this looks odd for the latex cm font.
     text = _replace_mathdefault(text)
+    text = text.replace("\N{MINUS SIGN}", r"\ensuremath{-}")
     # split text into normaltext and inline math parts
     parts = re_mathsep.split(text)
     for i, s in enumerate(parts):
@@ -791,10 +792,7 @@ def _print_pgf_to_fh(self, fh, *, bbox_inches_restore=None):
 %% Make sure the required packages are loaded in your preamble
 %%   \\usepackage{pgf}
 %%
-%% and, on pdftex
-%%   \\usepackage[utf8]{inputenc}\\DeclareUnicodeCharacter{2212}{-}
-%%
-%% or, on luatex and xetex
+%% and on luatex and xetex
 %%   \\usepackage{unicode-math}
 %%
 %% Figures using additional raster images can only be included by \\input if

lib/matplotlib/tests/test_backend_pgf.py

@@ -12,8 +12,10 @@
 import matplotlib as mpl
 import matplotlib.pyplot as plt
 from matplotlib.testing.compare import compare_images, ImageComparisonFailure
-from matplotlib.testing.decorators import image_comparison, _image_directories
 from matplotlib.backends.backend_pgf import PdfPages, common_texification
+from matplotlib.testing.decorators import (_image_directories,
+                                           check_figures_equal,
+                                           image_comparison)
 
 baseline_dir, result_dir = _image_directories(lambda: 'dummy func')
 
@@ -338,3 +340,12 @@ def test_unknown_font(caplog):
         plt.savefig(BytesIO(), format="pgf")
     assert "Ignoring unknown font: this-font-does-not-exist" in [
         r.getMessage() for r in caplog.records]
+
+
+@check_figures_equal(extensions=["pdf"])
+@pytest.mark.parametrize("texsystem", ("pdflatex", "xelatex", "lualatex"))
+@pytest.mark.backend("pgf")
+def test_minus_signs_with_tex(fig_test, fig_ref, texsystem):
+    mpl.rcParams["pgf.texsystem"] = texsystem
+    fig_test.text(.5, .5, "$-1$")
+    fig_ref.text(.5, .5, "$\N{MINUS SIGN}1$")