title: ggplot2, error in filling the area under lines
body: <p>I have this data set and I want to fill the <code>area under each line</code>. However I get an error saying:</p>
  
  <blockquote>
    <p>Error: stat_bin() must not be used with a y aesthetic.</p>
  </blockquote>
  
  <p>Additionally, I need to use <code>alpha</code> value for transparency. Any suggestions?</p>
  
  <pre><code>library(reshape2)
  library(ggplot2)
  
  dat &lt;- data.frame(
      a = rnorm(12, mean = 2, sd = 1),
      b = rnorm(12, mean = 4, sd = 2),
      month = c("JAN","FEB","MAR",'APR',"MAY","JUN","JUL","AUG","SEP","OCT","NOV","DEC"))
  
  dat$month &lt;- factor(dat$month, 
      levels = c("JAN","FEB","MAR",'APR',"MAY","JUN","JUL","AUG","SEP","OCT","NOV","DEC"), 
      ordered = TRUE)
  
  dat &lt;- melt(dat, id="month")
  
  ggplot(data = dat, aes(x = month, y = value, colour = variable)) + 
      geom_line() + 
      geom_area(stat ="bin")
  </code></pre>