title: Changing shapes used for scale_shape() in ggplot2
body: <p>Suppose I have the following</p>
  
  <pre><code>y &lt;- rnorm(10)
  b &lt;- as.factor(sample(1:4,10,replace=T))
  qplot(1:10, y, shape=b)
  </code></pre>
  
  <p>How do I change the shapes that are used using <code>ggplot2</code>?</p>