title: No handles with labels found to put in legend
body: <p>I'm trying to create a parallelogram in PyPlot. I'm not up to drawing the parallelogram--first I'm putting in the vector arrows--using the following code:</p>
  <pre><code>fig = plt.figure()
  ax = fig.add_subplot(111)
  ax.spines['left'].set_position('zero')
  ax.spines['right'].set_color('none')
  ax.spines['bottom'].set_position('zero')
  ax.spines['top'].set_color('none')
  plt.axis([-5,5,-5,5])
  ax.xaxis.set_ticks_position('bottom')
  ax.yaxis.set_ticks_position('left')
  plt.grid()
  plt.arrow(0,0, 3,1, head_width=0.2, color='r', length_includes_head=True, label='u')
  plt.arrow(0,0, 1,3, head_width=0.2, color='r', length_includes_head=True, label='v')
  plt.arrow(0,0, 4,4, head_width=0.2, color='r', length_includes_head=True, label='u+v')
  plt.legend()
  </code></pre>
  <p>This returns the following error:</p>
  <pre class="lang-none prettyprint-override"><code>No handles with labels found to put in legend.
  </code></pre>
  <p>I'm not sure why, because, based on the documentation for <code>plt.arrow()</code>, <code>label</code> is an acceptable kwarg, and <code>plt.legend()</code> should ostensibly be reading that. The rest of the figure draws fine; it's just missing the legend.</p>