title: pandas and matplotlib: Combine two plots into one legend item
body: <p>I have the following code:</p>
  
  <pre><code>from pandas import DataFrame
  import matplotlib.pyplot as plt
  
  if __name__ == '__main__':
      lower_bound = [0, 1, 2, 3]
      value = [1, 2, 3, 4]
      upper_bound = [2, 3, 4, 5]
      d = {'Lower bound': lower_bound, 'Value': value, 'Upper bound': upper_bound}
      df = DataFrame(data=d)
      ax = df.plot()
      plt.show()
  </code></pre>
  
  <p>It creates the following plot:</p>
  
  <p><a href="https://i.stack.imgur.com/MsCyX.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/MsCyX.png" alt="plot"></a></p>
  
  <p>I want the same plot, but, in the legend, I want the "Lower bound" and "Upper bound" combined into one, say, "Bounds".  And, of course, the two bounds will now be the same color (whatever color is given on the legend for "Bounds").</p>