title: overlay histogram by certain order of fill color
body: <p>When using geom_histogram in ggplot and position = "identity" is it possible to choose the order of which color goes first?</p>
  
  <pre><code>a &lt;- data.frame(rep(1:10, each = 4))
  b &lt;- data.frame(rep(1:3, each = 5))
  c &lt;- data.frame(rep(4:9, each = 3))
  names(a) &lt;- "num"
  names(b) &lt;- "num"
  names(c) &lt;- "num"
  a$color &lt;- "red"
  b$color &lt;- "white"
  c$color &lt;- "blue"
  
  abc &lt;- rbind(a,b,c)
  
  ggplot() + geom_histogram(data = abc, aes(x=num, fill = color),
  position = "identity") + scale_fill_identity()
  </code></pre>
  
  <p>Is it possible to plot white, then red then blue?</p>