title: ggplot2 density histogram with custom bin edges
body: <p>I'm able to plot a density histogram, and I'm able to plot a regular histogram with custom bins, but not both together. Here's my attempt:</p>
  
  <pre><code>library(ggplot2)
  
  vals = c(2.6, 5.2, 4.1, 6.9, 5.7, 5.2, 4.4, 5.5, 6.3, 6.1, 4.7, 1.4)
  myplot = qplot(vals, geom = 'blank') +   
           geom_line(aes(y = ..density..), stat = 'density',
                     colour = 26, size = 2, alpha = .6) +                     
           geom_histogram(aes(y = ..density..), binwidth = 1,
                          fill = 28, alpha = 0.3) +
           stat_bin(breaks=seq(-.5,8.5,1)) + xlim(-1, 9)
  
  print(myplot)
  </code></pre>
  
  <p>If you remove the <code>stat_bin</code> term, the histogram plots correctly as a density histogram, but with default bin locations. Add the <code>stat_bin</code> term, and the bins are correct but it's no longer a density histogram. Any ideas how to get both working?</p>