Support ~ as nonbreaking space in mathtext.

anntzer · anntzer · commit 97d8be588086 · 2018-12-07T11:27:51.000+01:00
... consistently with TeX. (AFAICT we never break mathtext over multiple lines so it's just a normal space for us.) See e.g. https://tex.stackexchange.com/a/74354/4101 for reference.
diff --git a/doc/api/next_api_changes/2018-12-07-AL.rst b/doc/api/next_api_changes/2018-12-07-AL.rst
@@ -0,0 +1,4 @@
+``~`` now interpreted as space in mathtext
+``````````````````````````````````````````
+In constructs such as ``"$1~2$"``, mathtext now interprets the tilde as a
+space, consistently with TeX (this was previously a parse error).
diff --git a/lib/matplotlib/mathtext.py b/lib/matplotlib/mathtext.py
@@ -2696,6 +2696,7 @@ def _make_space(self, percentage):
                       r'\:'         : 0.22222,   # 4/18 em = 4 mu
                       r'\;'         : 0.27778,   # 5/18 em = 5 mu
                       r'\ '         : 0.33333,   # 6/18 em = 6 mu
+                      r'~'          : 0.33333,   # 6/18 em = 6 mu, nonbreakable
                       r'\enspace'   : 0.5,       # 9/18 em = 9 mu
                       r'\quad'      : 1,         # 1 em = 18 mu
                       r'\qquad'     : 2,         # 2 em = 36 mu
