title: matplolib doesn't show legend starting with underscore _
body: <p>I have the following code in python :</p>
  
  <pre><code>import matplotlib.pyplot as plt
  plt.plot([1,2,3],label="_test")
  plt.legend()
  plt.show()
  </code></pre>
  
  <p>And when running it, the following warning appears :</p>
  
  <blockquote>
    <p>/usr/lib/pymodules/python2.7/matplotlib/axes.py:4486: UserWarning: No
    labeled objects found. Use label='...' kwarg on individual plots.<br>
    warnings.warn("No labeled objects found. "</p>
  </blockquote>
  
  <p>and no legend shows up.</p>
  
  <p>It appears, after trying to remove the '_' at the beginning of the label that the problem comes from here. </p>
  
  <p>Does anybody know why it is so ? Is it a known voluntary/logical feature, an inconvenience or a bug ? Is it reported anywhere, since i couldn't find anything about this on <a href="http://matplotlib.org/users/legend_guide.html" rel="noreferrer">http://matplotlib.org/users/legend_guide.html</a> ? Is it specific to the label of the legend or it occurs as well with some other labels (title, xylabels maybe ?)</p>
  
  <p>Thank you very much for your help</p>