title: Pandas secondary y axis for boxplots
body: <p>I'd like to use a secondary y-axis for some boxplots in pandas, but it doesn't seem available. </p>
  
  <pre><code>import numpy as np
  import pandas as pd
  
  data = np.random.random((10, 5))
  data[:,-1] += 10  # offset one column
  df = pd.DataFrame(data)
  </code></pre>
  
  <p>Now, using the default line plot it's easy enough to plot to a second y-axis:</p>
  
  <pre><code>df.plot(kind='line', secondary_y=[4])
  </code></pre>
  
  <p><img src="https://i.stack.imgur.com/zO59k.png" alt="normal line plot"></p>
  
  <p>But if I use boxplot style, it doesn't work:</p>
  
  <pre><code>df.plot(kind='box', secondary_y=[4])
  </code></pre>
  
  <p><img src="https://i.stack.imgur.com/SZBxQ.png" alt="box plot without second yaxis"></p>
  
  <p>Is there any way (maybe through matplotlib) I can get pandas to plot 2 axes for boxplot? Using the <code>boxplot()</code> method would be preferable, but I'll take <code>plot()</code>. Thanks.</p>