title: ggplot2: separate color scale per facet
body: <p>Intuitively I'm looking for something like: <code>facet_(scales="free_color")</code></p>
  
  <p>I do something like</p>
  
  <pre><code>p &lt;- ggplot(mpg, aes(year, displ, color=model)) + facet_wrap(~manufacturer)
  p + geom_jitter()
  </code></pre>
  
  <p>That is: plot 2d measurements from individuals(<code>model</code>) belonging to different species(<code>manufacturer</code>) faceted by a species, indicating the individual by color.
  The problem is that all individuals share the same color scale - so that the points in a facet have very similar colors.</p>
  
  <p>Using the group aesthetic with geom_line would solve the problem, but lines tell different story than dots.</p>
  
  <p>Another obvious solution would be to drop the faceting and draw a separate plot for each subset. (If this should be the only solution: are there any quick, smart or proven ways to do that?)</p>