title: Create unique legend for subplot with different columns in pandas
body: <p>I have to draw 2 subplots from 2 df that have different columns. I want to get common legend that contains all the columns of the two, similarly to the example below.</p>
  
  <pre><code>d1 = pd.DataFrame({ #without one
      'two' : [-1.,- 2.,- 3., -4.],
      'three' : [4., 3., 2., 1.],
      'four' : [4., 3., 4., 3.]})
  tot_1=d1.sum(axis=1)
  
  d2 = pd.DataFrame({'one' : [1., 2., 3., 4.],
      'two' : [4., 3., 3., 1.],
      'three' : [-1., -1., -3., -4.],
      'four' : [4., 3., 2., 1.]})
  
  tot_2=d2.sum(axis=1)
  
  
  fig, ax = plt.subplots(nrows=2, ncols=1)
  
  #plot 1
  d1.plot.area(stacked=True,legend=False,ax=ax[0])
  tot_1.plot(linestyle='-', color='black',legend=False,ax=ax[0])
  
  ###SECOND GRAPH####
  
  ax3 = ax[1].twiny()
  
  #plot 2
  d2.plot.area(stacked=True,legend=False,ax=ax[1],sharex=ax[0])
  tot_2.plot(linestyle='-',color='black',legend=False,ax=ax[1])
  
  plt.show()
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/Xtr86.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/Xtr86.png" alt="enter image description here"></a></p>
  
  <p>The prob is that the columns are not exactly the same in the two dataframes/plots  (same are in both some not)  and should make sure that the legend has all the columns and the colors in the two plots and the legend match.</p>
  
  <p>Even better (but not necessary) would be if I could chose the color for each column, for instance using a dictionary with <code>col_name:color</code> to pass </p>