title: Display frequency instead of count with geom_bar() in ggplot
body: <p>On <a href="http://www.cookbook-r.com/Graphs/Bar_and_line_graphs_%28ggplot2%29/" rel="noreferrer">this page</a>, they give the following example</p>
  
  <pre><code>library(ggplot2)
  library(reshape2)
  ggplot(data=tips, aes(x=day)) + geom_bar(stat="bin")
  </code></pre>
  
  <p>Instead of a count I'd like to have a frequency in y-axis. How can I achieve this?</p>