title: Fill being ignored with group + facet_wrap in ggplot2 / geom_bar
body: <p>I suspect I might be using group incorrectly here, but I can't seem to understand why the fill color is getting ignored in the example below.</p>
  
  <pre><code>df &lt;- data.frame(a = factor(c(1,1,2,2,1,2,1,2)), 
                   b = factor(c(1,2,3,4,5,6,7,2)), 
                   c = factor(c(1,2,1,2,1,2,1,2)))
  p &lt;- ggplot(df, aes(x=b)) +
    geom_bar(aes(y =  ..density.., group = c, fill=a), binwidth = 1) + 
    facet_wrap(~ c) +
    scale_y_continuous(labels = percent_format()) +
    scale_color_hue()
  p
  </code></pre>
  
  <p>Any help would be greatly appreciated.
  Thanks in advance,
  --JT</p>