title: How to specify different colors with ggplot
body: <pre><code>library(ggplot2)
  p &lt;- ggplot(mtcars, aes(wt, mpg))
  p + geom_point(size=4)
  </code></pre>
  
  <p>Suppose you have the above <code>scatterplot</code>. How can you specify that the points that are <code>&gt;= 25</code> mpg will be plotted <code>red</code>, the one <code>between 20 and 25</code> <code>green</code> and the <code>0-20</code> <code>blue</code>? </p>
  
  <p>Can this be done with <code>ggplot</code> specifically?</p>