title: Legend only shows one label when plotting with pandas
body: <p>I have two Pandas DataFrames that I'm hoping to plot in single figure. I'm using IPython notebook.</p>
  
  <p>I would like the legend to show the label for both of the DataFrames, but so far I've been able to get only the latter one to show. Also any suggestions as to how to go about writing the code in a more sensible way would be appreciated. I'm new to all this and don't really understand object oriented plotting.</p>
  
  <pre><code>%pylab inline
  import pandas as pd
  
  #creating data
  
  prng = pd.period_range('1/1/2011', '1/1/2012', freq='M')
  var=pd.DataFrame(randn(len(prng)),index=prng,columns=['total'])
  shares=pd.DataFrame(randn(len(prng)),index=index,columns=['average'])
  
  #plotting
  
  ax=var.total.plot(label='Variance')
  ax=shares.average.plot(secondary_y=True,label='Average Age')
  ax.left_ax.set_ylabel('Variance of log wages')
  ax.right_ax.set_ylabel('Average age')
  plt.legend(loc='upper center')
  plt.title('Wage Variance and Mean Age')
  plt.show()
  </code></pre>
  
  <p><img src="https://i.stack.imgur.com/ODceD.png" alt="Legend is missing one of the labels"></p>