title: Conditional colouring of a geom_smooth
body: <p>I'm analyzing a series that varies around zero. And to see where there are parts of the series with a tendency to be mostly positive or mostly negative I'm plotting a <code>geom_smooth</code>. I was wondering if it is possible to have the color of the smooth line be dependent on whether or not it is above or below 0. Below is some code that produces a graph much like what I am trying to create.</p>
  
  <pre><code>set.seed(5)
  r &lt;- runif(22, max = 5, min = -5)
  t &lt;- rep(-5:5,2)
  df &lt;- data.frame(r+t,1:22)
  colnames(df) &lt;- c("x1","x2")
  ggplot(df, aes(x = x2, y = x1)) + geom_hline() + geom_line() + geom_smooth()
  </code></pre>
  
  <p>I considered calculating the smoothed values, adding them to the df and then using a <code>scale_color_gradient</code>, but I was wondering if there is a way to achieve this in <code>ggplot</code> directly.</p>