title: Show standard devation using geom_smooth and ggplot
body: <p>We have some data which represents many model runs under different scenarios. For a single scenario, we'd like to display the smoothed mean, with the filled areas representing standard deviation at a particular point in time, rather than the quality of the fit of smooting.</p>
  
  <p>For example:</p>
  
  <pre><code>d &lt;- as.data.frame( rbind( cbind( 1:20, 1:20,1 ), cbind(1:20, -1:-20,2 ) ) )
  names(d)&lt;-c("Time","Value","Run")
  ggplot( d, aes(x=Time,y=Value) ) + geom_line( aes(group=Run) ) + geom_smooth()
  </code></pre>
  
  <p>produces a graph with two runs represented, and a smoothed mean, but even though the SD between the runs is increasing, the smoother's bars stay the same size. I'd like to make the surrounds of the smoother represent standard deviation at a given timestep.</p>
  
  <p>Is there a non-labour intensive way of doing this, given many different runs and output variables?</p>