title: Stacked curves in geom_area plot
body: <p>How to stack the graphs in the following example?</p>
  
  <pre><code>a&lt;-as.POSIXlt("2013-07-01 00:00:00",origin = "1960-01-01",tz="GMT")
  b&lt;-as.POSIXlt("2013-07-08 00:00:00",origin = "1960-01-01",tz="GMT")
  woche1&lt;-sample(seq(as.numeric(a),by=60*60,length.out=200),200,T)
  woche2&lt;-sample(seq(as.numeric(b),by=60*60,length.out=200),200,T)
  times&lt;-c(woche1,woche2)
  class(times)&lt;-c("POSIXt","POSIXct") 
  times&lt;-as.POSIXlt(times,origin = "1960-01-01",tz="GMT")
  key&lt;-sample(LETTERS[1:3],200,T)
  df&lt;-data.frame(times=times,order=factor(rep(1:2,each=100)), key=key)
  p&lt;-ggplot(df, aes(x=times))
  p&lt;-p + geom_area(aes(y = ..count.., fill = key, group = key),stat = "bin",position = 'stack')#,position = 'stack'
  p&lt;-p + facet_wrap( ~ order,scales="free_x")
  p
  </code></pre>