title: Secondary y in pandas histogram plot. Legend is gone missing
body: <p>I'm having trouble with the legends in a simple pandas plotting task. It seems the second legend is being hidden by the first graph.</p>
  <p>A sample of my df:</p>
  <pre><code>print(campaign.head(10))
  
      hour_read  hour_sent
  0        16.0         13
  1        14.0         13
  2        13.0         13
  3        14.0         13
  4         NaN         13
  5        13.0         13
  6        13.0         13
  7        16.0         13
  8        14.0         13
  9        16.0         13
  10       15.0         15
  11       16.0         15
  12       15.0         15
  13       15.0         15
  14       15.0         15
  </code></pre>
  <p>I can plot both these series on the same histogram, and I am using `secondary_y'.</p>
  <p>However I can't get the legend for <code>hour_read</code> to appear. No matter what I try with the legend options.</p>
  <pre><code>plt.figure(figsize=(15,8))
  plt.xlabel('Email sent vs open hour')
  
  campaign.hour_read.plot(kind='hist',alpha=0.8,  label='Hour Read')
  
  campaign.hour_sent.plot(kind='hist',alpha=0.8 ,secondary_y=True, label='Hour Sent')
  
  plt.legend(loc='upper right')
  
  
  plt.show()
  </code></pre>
  <p><a href="https://i.stack.imgur.com/o2gKg.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/o2gKg.png" alt="enter image description here" /></a></p>
  <p>I have also tried setting up separate axes first. And using <code>#ax1.legend(loc=1)</code> and <code>ax2.legend(loc=2)</code> with many different variations. I think the <code>hour sent</code> plot is completely covering the <code>hour_read</code> legend. How can I get around this?</p>