title: Set legend position when plotting a pandas dataframe with a second y-axis via pandas plotting interface
body: <p>I am plotting a pandas dataframe with a second y-axis via pandas plotting interface as described in the <a href="https://pandas.pydata.org/pandas-docs/stable/visualization.html#plotting-on-a-secondary-y-axis" rel="nofollow noreferrer">documentation</a> like this:</p>
  
  <pre><code>df = pd.DataFrame(np.random.randn(24*3, 3),
                    index=pd.date_range('1/1/2019', periods=24*3, freq='h'))
  df.columns = ['A (left)', 'B (right)', 'C (right)']
  ax = df.plot(secondary_y=['B (right)', 'C (right)'], mark_right=False)
  ax.set_ylabel('A scale')
  ax.right_ax.set_ylabel('BC scale')
  ax.legend(loc='upper right')
  plt.show()
  </code></pre>
  
  <p>which yields
  <a href="https://i.stack.imgur.com/JLEdx.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/JLEdx.png" alt="enter image description here"></a>
  As it  can be seen, the legend looses entries when I set the position using <code>ax.legend(loc='upper right')</code>.</p>
  
  <p>Does anyone know how I can set the legend position and keep all entries?</p>
  
  <p>Thanks in advance!</p>