title: Plotting multiple lines, in different colors, with pandas dataframe
body: <p>I have a dataframe that looks like the following</p>
  <pre><code>   color  x   y
  0    red  0   0
  1    red  1   1
  2    red  2   2
  3    red  3   3
  4    red  4   4
  5    red  5   5
  6    red  6   6
  7    red  7   7
  8    red  8   8
  9    red  9   9
  10  blue  0   0
  11  blue  1   1
  12  blue  2   4
  13  blue  3   9
  14  blue  4  16
  15  blue  5  25
  16  blue  6  36
  17  blue  7  49
  18  blue  8  64
  19  blue  9  81
  </code></pre>
  <p>I ultimately want two lines, one blue, one red.  The red line should essentially be y=x and the blue line should be y=x^2</p>
  <p>When I do the following:</p>
  <pre><code>df.plot(x='x', y='y')
  </code></pre>
  <p>The output is this:</p>
  <p><img src="https://i.stack.imgur.com/SlPtV.png" alt="" /></p>
  <p>Is there a way to make pandas know that there are two sets?  And group them accordingly.  I'd like to be able to specify the column <code>color</code> as the set differentiator</p>