title: plt.plot issue in pandas with categorical index DataFrame
body: <p>I have a DataFrame with categorical index like so:</p>
  
  <pre><code>import pandas as pd
  import matplotlib.pyplot as plt
  %matplotlib notebook
  
  accidents_by_day=pd.DataFrame({'num_accidents':[5659,5298,4917,4461,4181,4038,3985],
                             'weekday':[7,1,6,5,4,2,3]})
  
  weekday_map={1:'Sunday',2:'Monday',3:'Tuesday',4:'Wednesday',5:'Thursday',6:'Friday',7:'Saturday'}
  new_index=(pd.CategoricalIndex(accidents_by_day.weekday.map(weekday_map)).
         reorder_categories(new_categories=['Monday','Tuesday','Wednesday','Thursday',
                                            'Friday','Saturday','Sunday'],
                            ordered=True))
  accidents_by_day.set_index(new_index,drop=True,inplace=True)
  accidents_by_day.sort_index(inplace=True)
  </code></pre>
  
  <p>While The following works fine:</p>
  
  <pre><code>accidents_by_day.num_accidents.plot(kind='bar')
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/Mgccb.png" rel="nofollow noreferrer"></a></p>
  
  <p>The <code>plt.plot(accidents_by_day.num_accidents)</code> gives an error </p>
  
  <pre><code>~\Anaconda3\lib\site-packages\matplotlib\axes\_base.py in _plot_args(self, tup, kwargs)
      390             func = self._makefill
      391 
  --&gt; 392         ncx, ncy = x.shape[1], y.shape[1]
      393         for j in xrange(max(ncx, ncy)):
      394             seg = func(x[:, j % ncx], y[:, j % ncy], kw, kwargs)
  
  IndexError: tuple index out of range
  </code></pre>
  
  <p>and <code>plt.plot([accidents_by_day.num_accidents])</code> produces an empty figure. </p>
  
  <p>Could anyone explain what is happening here?</p>
  
  <p>Thanks!</p>