title: ggplot2 labels for color=factor(foo)
body: <p>Having a hard time with labels on a ggplot2 plot. Here's a similar plot from the examples page:</p>
  
  <pre><code>mt &lt;- ggplot(mtcars, aes(mpg, wt, colour = factor(cyl))) + geom_point() 
  mt + facet_grid(. ~ cyl, scales = "free")
  </code></pre>
  
  <p>How would I define a list of labels for factor(cyl)?</p>