title: ggplot2 annotate using a measure unit other than the axis variable
body: <p>Hi I've got the following code</p>
  
  <pre><code>d1=data.frame(a=c(4,5,6,7),b=as.Date(c('2005-12-31','2006-12-31','2007-12-31','2008-12-31'),"%Y-%m-%d"))
  a = ggplot(d1,aes(x=b,y=a)) + geom_line()
  a + annotate('text',x=as.Date('2006-12-31','%Y-%m-%d'),y=5.5,label='blah')
  </code></pre>
  
  <p>But annotating the graph is really clunky. I'd like to be able to specify the x axis using percentage of axis (for example) or inches or something else. Is this possible and how would I go about it?</p>
  
  <p>Thanks</p>