title: use ggplot2 to plot two lines with ribbons
body: <p>I have a data frame with a metric v and a categorical variable t.
  I can plot them as lines like this</p>
  
  <pre><code>set.seed(42)
  v &lt;- runif(20, min=0, max=100)
  t &lt;- sample( LETTERS[1:2], 20, replace=TRUE )
  
  df &lt;- data.frame(v,t)
  
  qplot(1:length(v),v,data=df,geom="line",group=t,color=t)
  </code></pre>
  
  <p>I'd like to add ribbons around each line. Possibly of different width (that I'll set with a formula) and transparency. I tried to replace the geom with "ribbon" but I get the following error:</p>
  
  <pre><code>qplot(1:length(v),v,data=df,geom="ribbon",group=t,color=t)
  Error: geom_ribbon requires the following missing aesthetics: ymin, ymax
  </code></pre>
  
  <p>How can I plot both lines and their ribbons in one chart?</p>