Fix matplotlib#5489: feature for specifying N in color cycle

examples/pylab_examples/color_demo.py

@@ -1,6 +1,6 @@
 #!/usr/bin/env python
 """
-matplotlib gives you 4 ways to specify colors,
+matplotlib gives you 5 ways to specify colors,
 
     1) as a single letter string, ala MATLAB
 
@@ -11,6 +11,10 @@
     4) as a string representing a floating point number
        from 0 to 1, corresponding to shades of gray.
 
+    5) as a string representing the Nth color in the
+       currently active color cycle, e.g. "[0]"
+       represents the first color in the color cycle.
+
 See help(colors) for more info.
 """
 import matplotlib.pyplot as plt
@@ -20,8 +24,8 @@
 #subplot(111, facecolor='#ababab')
 t = np.arange(0.0, 2.0, 0.01)
 s = np.sin(2*np.pi*t)
-plt.plot(t, s, 'y')
-plt.xlabel('time (s)', color='r')
+plt.plot(t, s, '[0]')
+plt.xlabel('time (s)', color='[0]')
 plt.ylabel('voltage (mV)', color='0.5')  # grayscale color
 plt.title('About as silly as it gets, folks', color='#afeeee')
 plt.show()

lib/matplotlib/axes/_base.py

@@ -10,6 +10,7 @@
 import warnings
 import math
 from operator import itemgetter
+import re
 
 import numpy as np
 from numpy import ma
@@ -94,6 +95,17 @@ def _process_plot_format(fmt):
         linestyle = 'None'
         fmt = fmt.replace(' ', '')
 
+    # handle the N-th color in the color cycle
+    matches = re.findall('\[[0-9]+\]', fmt)
+    if len(matches) == 1:
+        color = mcolors.colorConverter._get_nth_color(
+            int(matches[0][1:-1]))
+        fmt = fmt.replace(matches[0], '')
+    elif len(matches) != 0:
+        if color is not None:
+            raise ValueError(
+                'Illegal format string "%s"; two color symbols' % fmt)
+
     chars = [c for c in fmt]
 
     for c in chars:

lib/matplotlib/colors.py

@@ -31,6 +31,13 @@
     - k: black
     - w: white
 
+To use the colors that are part of the active color cycle in the
+current style, use a string with the color number in square brackets.
+For example:
+
+    - `[0]`: The first color in the cycle
+    - `[1]`: The second color in the cycle
+
 Gray shades can be given as a string encoding a float in the 0-1 range, e.g.::
 
     color = '0.75'
@@ -260,8 +267,36 @@ class ColorConverter(object):
         'k': (0.0, 0.0, 0.0),
         'w': (1.0, 1.0, 1.0), }
 
+    _prop_cycler = None
+
     cache = {}
 
+    @classmethod
+    def _get_nth_color(cls, val):
+        """
+        Get the Nth color in the current color cycle.  If N is greater
+        than the number of colors in the cycle, it is wrapped around.
+        """
+        from matplotlib.rcsetup import cycler
+        from matplotlib import rcParams
+
+        prop_cycler = rcParams['axes.prop_cycle']
+        if prop_cycler is None and 'axes.color_cycle' in rcParams:
+            clist = rcParams['axes.color_cycle']
+            prop_cycler = cycler('color', clist)
+
+        cycler = cls._get_property_cycler()
+        colors = cycler.by_key()['color']
+        return colors[val % len(colors)]
+
+    @classmethod
+    def _parse_nth_color(cls, val):
+        match = re.match('^\[[0-9]\]$', val)
+        if match is not None:
+            return cls._get_nth_color(int(val[1:-1]))
+
+        raise ValueError("Not a color cycle color")
+
     def to_rgb(self, arg):
         """
         Returns an *RGB* tuple of three floats from 0-1.
@@ -299,6 +334,10 @@ def to_rgb(self, arg):
                 argl = arg.lower()
                 color = self.colors.get(argl, None)
                 if color is None:
+                    try:
+                        argl = self._parse_nth_color(argl)
+                    except ValueError:
+                        pass
                     str1 = cnames.get(argl, argl)
                     if str1.startswith('#'):
                         color = hex2color(str1)