title: setting a legend matching the colours in pyplot.scatter
body: <p>Suppose my data is organized in the following way:</p>
  
  <pre><code>x_values = [6.2, 3.6, 7.3, 3.2, 2.7]
  y_values = [1.5, 3.2, 5.4, 3.1, 2.8]
  colours = [1, 1, 0, 1, -1]
  labels = ["a", "a", "b", "a", "c"]
  </code></pre>
  
  <p>I want to make a scatterplot with this:</p>
  
  <pre><code>axis = plt.gca()
  axis.scatter(x_values, y_values, c=colours)
  </code></pre>
  
  <p>I want a legend with 3 categories: "a", "b" and "c".</p>
  
  <p>Can I use the <code>labels</code> list to make this legend, given that the categories in this list match the order of the points in the <code>colours</code> list?</p>
  
  <p>Do I need to run the <code>scatter</code> command separately for each category?</p>