ggplot2, error in filling the area under lines
<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>