title: NaN-columns is plotted as a all 0 column in pandas
body: <p>I have got some problems with plotting a sliced <code>DataFrame</code> with entire columns filled with NaN's.</p>
  
  <p>How come:</p>
  
  <pre><code>pandas.DataFrame(
      dict(
          A=pandas.Series([np.NaN]*32),
          B=pd.Series(range(-1,32))
      )
  ).plot()
  </code></pre>
  
  <p>differs from:</p>
  
  <pre><code>#Ugly fix
  pandas.DataFrame(
      dict(
          A=pandas.Series( [0] + [numpy.NaN]*32),
          B=pd.Series(range(-1,32))
      )
  ).plot()
  </code></pre>
  
  <p>by plotting a 0-line as if the column is filled with zeros.
  Shouldn't the first code work just as:</p>
  
  <pre><code>pylab.plot(
      range(0,33),
      range(-1,32),
      range(0,32),
      [numpy.NaN]*32
  )
  </code></pre>
  
  <p>And also plotting just a Series filled with NaN works fine:</p>
  
  <pre><code>pandas.Series([numpy.NaN]*32).plot()
  </code></pre>
  
  <p>What am I missing? Is there a right way to plot a column with all NaN's or is it a bug?</p>