title: Matplotlib scatterplot; color as a function of a third variable
body: <p>I want to make a scatterplot (using matplotlib) where the points are shaded according to a third variable. I've got very close with this:</p>
  <pre><code>plt.scatter(w, M, c=p, marker='s')
  </code></pre>
  <p>where w and M are the data points and p is the variable I want to shade with respect to.<br />
  However I want to do it in greyscale rather than colour. Can anyone help?</p>