Fixed decimal points not appearing at end of Mathtext string. (matplotlib#23357)

* Fixed decimal points not appearing at end of Mathtext string.

* Added test for Mathtext strings ending with decimal point.

Author: tfpf

lib/matplotlib/_mathtext.py

@@ -2000,16 +2000,16 @@ def symbol(self, s, loc, toks):
                                self._make_space(0.2)],
                               do_kern=True)]
         elif c in self._punctuation_symbols:
+            prev_char = next((c for c in s[:loc][::-1] if c != ' '), '')
+            next_char = next((c for c in s[loc + 1:] if c != ' '), '')
 
             # Do not space commas between brackets
             if c == ',':
-                prev_char = next((c for c in s[:loc][::-1] if c != ' '), '')
-                next_char = next((c for c in s[loc + 1:] if c != ' '), '')
                 if prev_char == '{' and next_char == '}':
                     return [char]
 
             # Do not space dots as decimal separators
-            if c == '.' and s[loc - 1].isdigit() and s[loc + 1].isdigit():
+            if c == '.' and prev_char.isdigit() and next_char.isdigit():
                 return [char]
             else:
                 return [Hlist([char, self._make_space(0.2)], do_kern=True)]

lib/matplotlib/tests/baseline_images/test_mathtext/mathtext1_dejavusans_03.png

@@ -128,6 +128,7 @@
     r'$\sqrt[ab]{123}$',  # github issue #8665
     r'$x \overset{f}{\rightarrow} \overset{f}{x} \underset{xx}{ff} \overset{xx}{ff} \underset{f}{x} \underset{f}{\leftarrow} x$',  # github issue #18241
     r'$\sum x\quad\sum^nx\quad\sum_nx\quad\sum_n^nx\quad\prod x\quad\prod^nx\quad\prod_nx\quad\prod_n^nx$',  # GitHub issue 18085
+    r'$1.$ $2.$ $19680801.$ $a.$ $b.$ $mpl.$',
 ]
 
 digits = "0123456789"