title: How to set multiple colours in a ggplot2 stat_summary plot?
body: <p>How do I use <code>ggplot2</code> with <code>stat_summary</code> to show colours of my choice? Eg.:</p>
  
  <pre><code>simVol  &lt;- data.frame(simId=c(1,1,1,1,2,2,2,2), 
                        farm=rep(c('farm A', 'farm A', 'farm B', 'farm B'),2),
                        period=rep(1:2,4), 
                        volume=c(9,21,12,18,10,22,11,19))
  
  P10meanP90 &lt;- function(x) data.frame(
    y = mean(x), 
    ymin = quantile(x, .1),
    ymax = quantile(x, .9)
  )
  </code></pre>
  
  <p>This command plots the distribution of volume at each farm against the period, using default colours:</p>
  
  <pre><code>ggplot(simVol, aes(x=period, y=volume, colour=farm)) + 
      stat_summary(fun.data="P10meanP90", geom="smooth", size=2)
  </code></pre>
  
  <p>However, if I add <code>colour='green'</code> to the arguments of <code>stat_summary</code>, it plots instead the aggregate across farms. I've tried using <code>colour=c('green','orange')</code>, but this still only shows a green line.</p>
  
  <p>How do I change the colours in this plot?</p>
  
  <p>thanks</p>