title: What does the matplotlib `hold` keyword argument do?
body: <p>The <code>hold</code> argument appears in many plotting functions, for example <code>plt.fill_between</code>, <code>plt.arrow</code>, <code>plt.axhline</code> (for <code>import matplotlib.pyplot as plt</code>).  I can't find any documentation on it, however.  <a href="http://matplotlib.org/api/pyplot_api.html#matplotlib.pyplot.axhspan">Docs seem to only state</a>:</p>
  
  <blockquote>
    <p>Additional kwargs: hold = [True|False] overrides default hold state</p>
  </blockquote>
  
  <p>This is also a difficult keyword argument to google for...</p>