Support Cn colors with n>=10.

anntzer · anntzer · commit b5eba123707a · 2018-10-23T14:25:51.000+02:00
While the change is mildly backwards-incompatible (as noted in the API
changes), Cn syntax is by far the simplest way to navigate the
color-cycle, so I think the change is worth it.
diff --git a/doc/api/next_api_changes/2018-10-23-AL.rst b/doc/api/next_api_changes/2018-10-23-AL.rst
@@ -0,0 +1,12 @@
+``Cn`` colors now support ``n>=10``
+```````````````````````````````````
+
+It is now possible to go beyond the tenth color in the property cycle using
+``Cn`` syntax, e.g. ``plt.plot([1, 2], color="C11")`` now uses the 11th color
+in the cycle.
+
+Note that previously, a construct such as ``plt.plot([1, 2], "C11")`` would be
+interpreted as a request to use color ``C1`` and marker ``1`` (an "inverted Y").
+To obtain such a plot, one should now use ``plt.plot([1, 2], "1C1")`` (so that
+the first "1" gets correctly interpreted as a marker specification), or, more
+explicitly, ``plt.plot([1, 2], marker="1", color="C1")``.
diff --git a/lib/matplotlib/colors.py b/lib/matplotlib/colors.py
@@ -115,7 +115,7 @@ def _sanitize_extrema(ex):
 
 def _is_nth_color(c):
     """Return whether *c* can be interpreted as an item in the color cycle."""
-    return isinstance(c, str) and re.match(r"\AC[0-9]\Z", c)
+    return isinstance(c, str) and re.match(r"\AC[0-9]+\Z", c)
 
 
 def is_color_like(c):
@@ -169,7 +169,7 @@ def to_rgba(c, alpha=None):
         from matplotlib import rcParams
         prop_cycler = rcParams['axes.prop_cycle']
         colors = prop_cycler.by_key().get('color', ['k'])
-        c = colors[int(c[1]) % len(colors)]
+        c = colors[int(c[1:]) % len(colors)]
     try:
         rgba = _colors_full_map.cache[c, alpha]
     except (KeyError, TypeError):  # Not in cache, or unhashable.
diff --git a/lib/matplotlib/tests/test_colors.py b/lib/matplotlib/tests/test_colors.py
@@ -637,6 +637,8 @@ def test_cn():
                                                     ['xkcd:blue', 'r'])
     assert mcolors.to_hex("C0") == '#0343df'
     assert mcolors.to_hex("C1") == '#ff0000'
+    assert mcolors.to_hex("C10") == '#0343df'
+    assert mcolors.to_hex("C11") == '#ff0000'
 
     matplotlib.rcParams['axes.prop_cycle'] = cycler('color', ['8e4585', 'r'])
 
