title: Plots coming out empty with pandas.plot
body: <p>I am trying to plot a df that must have two y-axes. I can get the plot to work using only one axis, but when I use two it comes out empty. I've tried separating into two separate dataframes and equally not doing this but neither is working.</p>
  
  <p>My code currently:</p>
  
  <pre><code>    df1 = A dataframe with two columns of data and a period index.
      df2 = A dataframe with one column of data and a period index, to 
      plot on a separate axis .
  
      colors = ['b', 'g']            
      styles = ['-', '-']
      linewidths = [4,2]
  
      fig, ax = plt.subplots()
      for col, style, lw, color in zip(df1.columns, styles, linewidths, colors):
          df1[col].plot(style=style, color=color, lw=lw, ax=ax)
  
      plt.xlabel('Date')
  
      plt.ylabel('First y axis label')
      plt.hold()
  
      colors2 = ['b']
      styles2 = ['-']
      fig2, ax2 = plt.subplots()
  
      for col, style, lw, color in zip(df2.columns, styles, linewidths, colors):
          df2.monthly_windspeed_to_plot[col].plot(style=style, color=color, lw=lw, ax=ax)
      plt.ylabel('Second y axis label')
  
      plt.title('A Title')
      plt.legend(['Item 1', 'Item 2', 'Item 3'], loc='upper center',
                  bbox_to_anchor=(0.5, 1.05))
  
      plt.savefig("My title.png")
  </code></pre>
  
  <p>The result of this is an empty plot.</p>
  
  <p>What is the mistake in my code?</p>