title: Don't drop zero count: dodged barplot
body: <p>I am making a dodged barplot in ggplot2 and one grouping has a zero count that I want to display.  I remembered seeing this on <a href="http://groups.google.com/group/ggplot2/browse_thread/thread/b520e0be08f5d100" rel="noreferrer">HERE</a> a while back and figured the <code>scale_x_discrete(drop=F)</code> would work.  It does not appear to work with dodged bars.  How can I make the zero counts show?</p>
  
  <p>For instance, (code below) in the plot below, type8~group4 has no examples. I would still like the plot to display the empty space for the zero count instead of eliminating the bar.  How can I do this?</p>
  
  <p><img src="https://i.stack.imgur.com/uuvcJ.png" alt="enter image description here"></p>
  
  <pre><code>mtcars2 &lt;- data.frame(type=factor(mtcars$cyl), 
      group=factor(mtcars$gear))
  
  m2 &lt;- ggplot(mtcars2, aes(x=type , fill=group))
  p2 &lt;- m2 + geom_bar(colour="black", position="dodge") +
          scale_x_discrete(drop=F)
  p2
  </code></pre>