title: How to set linetype for geom_vline in a faceted plot?
body: <p>The following code works like a charm:</p>
  
  <pre><code>foo = data.frame(x=c(1,2,3),
                   y=c(4,5,6),
                   lt=c('dotted', 'dashed', 'dotdash'))
  ggplot(foo, aes(x,y)) + geom_point() +
    geom_vline(aes(xintercept=x, linetype=lt))
  </code></pre>
  
  <p><img src="https://i.stack.imgur.com/DYQj0.png" alt="enter image description here"></p>
  
  <p>The following code drops linetype.  Why?</p>
  
  <pre><code>ggplot(foo, aes(x,y)) + geom_point() +
    geom_vline(xintercept=3, aes(linetype=lt)) +
    facet_wrap(~lt)
  </code></pre>
  
  <p><img src="https://i.stack.imgur.com/skqEU.png" alt="enter image description here"></p>
  
  <p>I am using ggplot 0.9.3.1 in Rstudio.</p>