title: Use datetime.date in matplotlib with Python3
body: <p>I try to plot tuple-values. There is one float for each day.
  I tried this:</p>
  
  <pre><code>#!/usr/bin/env python3
  # -*- coding: utf-8 -*-
  
  import matplotlib.pyplot as plt
  import datetime
  
  d = tuple()
  d += (70.3, datetime.date(2015, 1, 7))
  d += (60.1, datetime.date(2015, 2, 5))
  d += (68.8, datetime.date(2015, 6, 2))
  
  plt.plot(d)
  plt.show()
  </code></pre>
  
  <p>This doesn't work because of</p>
  
  <pre><code>&gt; Traceback (most recent call last):   File "./m.py", line 12, in
  &gt; &lt;module&gt;
  &gt;     plt.plot(d)   File "/usr/local/lib/python3.4/dist-packages/matplotlib-1.5.dev1-py3.4-linux-i686.egg/matplotlib/pyplot.py",
  &gt; line 3129, in plot
  &gt;     ret = ax.plot(*args, **kwargs)   File "/usr/local/lib/python3.4/dist-packages/matplotlib-1.5.dev1-py3.4-linux-i686.egg/matplotlib/axes/_axes.py",
  &gt; line 1382, in plot
  &gt;     self.add_line(line)   File "/usr/local/lib/python3.4/dist-packages/matplotlib-1.5.dev1-py3.4-linux-i686.egg/matplotlib/axes/_base.py",
  &gt; line 1568, in add_line
  &gt;     self._update_line_limits(line)   File "/usr/local/lib/python3.4/dist-packages/matplotlib-1.5.dev1-py3.4-linux-i686.egg/matplotlib/axes/_base.py",
  &gt; line 1579, in _update_line_limits
  &gt;     path = line.get_path()   File "/usr/local/lib/python3.4/dist-packages/matplotlib-1.5.dev1-py3.4-linux-i686.egg/matplotlib/lines.py",
  &gt; line 908, in get_path
  &gt;     self.recache()   File "/usr/local/lib/python3.4/dist-packages/matplotlib-1.5.dev1-py3.4-linux-i686.egg/matplotlib/lines.py",
  &gt; line 609, in recache
  &gt;     y = np.asarray(yconv, np.float_)   File "/usr/local/lib/python3.4/dist-packages/numpy/core/numeric.py", line
  &gt; 462, in asarray
  &gt;     return array(a, dtype, copy=False, order=order) TypeError: float() argument must be a string or a number, not 'datetime.datetime'
  </code></pre>
  
  <p>But looking into the matplotlib docu datetime should be full supported.</p>