title: Difference between two lines ggplot2
body: <p>I wanted to add the difference between two lines to a <code>ggplot2</code>.
  In this example, between the lines of the two groups defined by <code>x2</code>. 
  How can this be done? </p>
  
  <pre><code>y=rbinom(100,1,.4)
  x1=rnorm(100, 3, 2)
  x2=rbinom(100, 1, .7) 
  sub = data.frame(y=y, x1=x1, x2=x2)
  
  ggplot(sub, aes(x1, y, color = x2))   + 
        stat_smooth(method = "glm", family = binomial, formula = y ~ poly(x1,3))
  </code></pre>