Let FormatStrFormatter respect axes.unicode_minus.

This is better than the workaround of wrapping the expression in
mathtext (as suggested for the old-style FormatStrFormatter) because it
also works for the hover tooltip in the toolbar (which cannot display
mathtext).

Note that it is not as easy to do so for FormatStrFormatter
(%-formatting) as that would require manually parsing the format string;
I would not bother as that formatter seems to be mostly present for
backcompat.

Author: anntzer

doc/users/next_whats_new/formatter_unicode_minus.rst

@@ -0,0 +1,4 @@
+``StrMethodFormatter`` now respects ``axes.unicode_minus``
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+When formatting negative values, `.StrMethodFormatter` will now use unicode
+minus signs if :rc:`axes.unicode_minus` is set.

lib/matplotlib/tests/test_ticker.py

@@ -1434,14 +1434,21 @@ def test_basic(self):
 
 class TestStrMethodFormatter:
     test_data = [
-        ('{x:05d}', (2,), '00002'),
-        ('{x:03d}-{pos:02d}', (2, 1), '002-01'),
+        ('{x:05d}', (2,), False, '00002'),
+        ('{x:05d}', (2,), True, '00002'),
+        ('{x:05d}', (-2,), False, '-0002'),
+        ('{x:05d}', (-2,), True, '\N{MINUS SIGN}0002'),
+        ('{x:03d}-{pos:02d}', (2, 1), False, '002-01'),
+        ('{x:03d}-{pos:02d}', (2, 1), True, '002-01'),
+        ('{x:03d}-{pos:02d}', (-2, 1), False, '-02-01'),
+        ('{x:03d}-{pos:02d}', (-2, 1), True, '\N{MINUS SIGN}02-01'),
     ]
 
-    @pytest.mark.parametrize('format, input, expected', test_data)
-    def test_basic(self, format, input, expected):
-        fmt = mticker.StrMethodFormatter(format)
-        assert fmt(*input) == expected
+    @pytest.mark.parametrize('format, input, unicode_minus, expected', test_data)
+    def test_basic(self, format, input, unicode_minus, expected):
+        with mpl.rc_context({"axes.unicode_minus": unicode_minus}):
+            fmt = mticker.StrMethodFormatter(format)
+            assert fmt(*input) == expected
 
 
 class TestEngFormatter:

lib/matplotlib/ticker.py

@@ -135,6 +135,7 @@
 import locale
 import math
 from numbers import Integral
+import string
 
 import numpy as np
 
@@ -353,16 +354,27 @@ def __call__(self, x, pos=None):
         return self.fmt % x
 
 
+class _UnicodeMinusFormat(string.Formatter):
+    """
+    A specialized string formatter so that `.StrMethodFormatter` respects
+    :rc:`axes.unicode_minus`.  This implementation relies on the fact that the
+    format string is only ever called with kwargs *x* and *pos*, so it blindly
+    replaces dashes by unicode minuses without further checking.
+    """
+
+    def format_field(self, value, format_spec):
+        return Formatter.fix_minus(super().format_field(value, format_spec))
+
+
 class StrMethodFormatter(Formatter):
     """
     Use a new-style format string (as used by `str.format`) to format the tick.
 
     The field used for the tick value must be labeled *x* and the field used
     for the tick position must be labeled *pos*.
 
-    Negative numeric values (e.g., -1) will use a dash, not a Unicode minus;
-    use mathtext to get a Unicode minus by wrapping the format specifier with $
-    (e.g. "${x}$").
+    The formatter will respect :rc:`axes.unicode_minus` when formatting
+    negative numeric values.
 
     It is typically unnecessary to explicitly construct `.StrMethodFormatter`
     objects, as `~.Axis.set_major_formatter` directly accepts the format string
@@ -379,7 +391,7 @@ def __call__(self, x, pos=None):
         *x* and *pos* are passed to `str.format` as keyword arguments
         with those exact names.
         """
-        return self.fmt.format(x=x, pos=pos)
+        return _UnicodeMinusFormat().format(self.fmt, x=x, pos=pos)
 
 
 class ScalarFormatter(Formatter):