title: Overlaying histograms with ggplot2 in R
body: <p>I am new to R and am trying to plot 3 histograms onto the same graph.
  Everything worked fine, but my problem is that you don't see where 2 histograms overlap - they look rather cut off.</p>
  <p>When I make density plots, it looks perfect: each curve is surrounded by a black frame line, and colours look different where curves overlap.</p>
  <p>Can someone tell me if something similar can be achieved with the histograms in the 1st picture? This is the code I'm using:</p>
  <pre><code>lowf0 &lt;-read.csv (....)
  mediumf0 &lt;-read.csv (....)
  highf0 &lt;-read.csv(....)
  lowf0$utt&lt;-'low f0'
  mediumf0$utt&lt;-'medium f0'
  highf0$utt&lt;-'high f0'
  histogram&lt;-rbind(lowf0,mediumf0,highf0)
  ggplot(histogram, aes(f0, fill = utt)) + geom_histogram(alpha = 0.2)
  </code></pre>