title: R ggplot extend the range of a category x axis
body: <p>If I have a dataframe similar to that below</p>
  
  <pre><code>a=data.frame(year=paste('FY',2001:2012,sep='.'),values=rnorm(12))
  library(ggplot2)
  </code></pre>
  
  <p>The following graph works </p>
  
  <pre><code>ggplot(a,aes(x=year,y=values,group=1))+geom_line() 
  </code></pre>
  
  <p>but the following one does not.</p>
  
  <pre><code>ggplot(a,aes(x=year,y=values,group=1))+geom_line() +xlim(0,13)
  </code></pre>
  
  <p>How can I extend the limits of a ggplot of data that has a category axis rather than a numeric one?</p>