title: formatting secondary y axis in pandas
body: <p>I am plotting a pandas DataFrame with several columns as below:</p>
  
  <pre><code>fig, ax = py.subplots(figsize=(11.7, 8.3))
  df.plot(ax=ax, secondary_y=[A])
  </code></pre>
  
  <p>I can format the primary yaxis with a command as below:</p>
  
  <pre><code>ax.yaxis.set_major_formatter(FormatStrFormatter('%d days'))
  </code></pre>
  
  <p>How can I apply formatting to the secondary Y-axis (the one that displays on the right)? </p>