title: Matplot histogram results in lots of lines
body: <p>I am trying to plot a histogram of monthly return data.  There are 78 observations, preceded by 73 blanks (in Excel) or NaNs in the DataFrame.  When I try and plot the histogram it results in a figure with lots of vertical lines going from 0.0 to 1.0 with colourful lines evenly spaced between (which I take to mean I'm doing something wrong).</p>
  
  <p>The DataFrame that I'm working with looks like this:</p>
  
  <pre><code>date
  2009-03-31       NaN
  2009-04-30       NaN
  2009-05-31       NaN
                 ...  
  2017-01-31     0.45%
  2017-02-28     3.33%
  2017-03-31    -1.55%
  </code></pre>
  
  <p>I've tried re-specifying every argument in the plt.hist() function, different numbers of bins, different ranges, and some forms of "dropna", but haven't been able to resolve.</p>
  
  <pre><code>df['column'].hist(bins=20)
  plt.plot
  </code></pre>
  
  <p>I'm expecting a distribution of monthly returns ranging from around -10% to +10% which is somewhat normal.  What I actually get is just a grid with fine vertical lines, with colourful lines evenly distributed (I assume these are the bins).  There is no error message.</p>