title: Making `matplotlib.pyplot.plot()` work with `PeriodIndex
body: <p>I have a DataFrame (<code>df</code>) that has a PeriodIndex and corresponding column of integers (<code>OrderQuantity</code>). (Think of this as data about quantities sold in a day for a particular product). In order to do some simple visualization of this data using <code>matplotlib.pyplot.plot()</code>, I am having to convert the PeriodIndex into Timestamp, otherwise the <code>plot()</code> method throws an error <code>TypeError: Axis must have</code>freq<code>set to convert to Periods</code>. It appears MatPlotLib tries to convert the PeriodIndex into  Periods, without realizing that it already has Periods.</p>
  
  <p>So, for now, the following works:</p>
  
  <pre><code>import matplotlib.pyplot as plt
  %matplotlib inline
  fig = plt.figure(figsize=(15,8))
  plt.plot(df.index.to_timestamp(), df.OrderQuantity) // produces a valid plot
  </code></pre>
  
  <p>I am wondering if there is a better way of plotting Periods without having to convert to a Timestamp, which pyplot will internally convert back to a Periods before generating the graph.</p>