Support ~ as nonbreaking space in mathtext.

... 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.

Author: anntzer

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).

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

lib/matplotlib/tests/test_mathtext.py

@@ -5,7 +5,7 @@
 import pytest
 
 import matplotlib
-from matplotlib.testing.decorators import image_comparison
+from matplotlib.testing.decorators import check_figures_equal, image_comparison
 import matplotlib.pyplot as plt
 from matplotlib import mathtext
 
@@ -271,3 +271,9 @@ def test_single_minus_sign():
 
     # If this fails, it would be all white
     assert not np.all(array == 0xff)
+
+
+@check_figures_equal(extensions=["png"])
+def test_spaces(fig_test, fig_ref):
+    fig_test.subplots().set_title(r"$1\,2\>3\ 4$")
+    fig_ref.subplots().set_title(r"$1\/2\:3~4$")