title: Multiple vlines in plot gives error, ggplot2
body: <p>I want to do exactly as explained <a href="https://stackoverflow.com/questions/5388832/how-to-get-a-vertical-geom-vline-to-an-x-axis-of-class-date/38078898?noredirect=1#comment63598027_38078898">here</a>, the accepted answer does for some reason not work for me.</p>
  
  <pre><code>library("ggplot2")
  
  tmp &lt;- data.frame(x=rep(seq(as.Date(0, origin="1970-01-01"),
                              length=36, by="1 month"), 2),
                    y=rnorm(72),
                    category=gl(2,36))
  
  p &lt;- ggplot(tmp, aes(x, y, colour=category)) +
       geom_line() +
       geom_vline(aes(xintercept=as.numeric(x[c(13, 24)])),
                  linetype=4, colour="black")
  print(p)
  </code></pre>
  
  <p>I exchanged layers(geom="line") with geom_line() to get only the plot working. However, when adding vlines I get this error:</p>
  
  <pre><code>Error: Aesthetics must be either length 1 or the same as the data (72):  xintercept
  10: stop("Aesthetics must be either length 1 or the same as the data (", 
      n, "): ", paste(names(!good), collapse = ", "), call. = FALSE)
  9: check_aesthetics(evaled, n)
  8: f(..., self = self)
  7: l$compute_aesthetics(d, plot)
  6: f(l = layers[[i]], d = data[[i]])
  5: by_layer(function(l, d) l$compute_aesthetics(d, plot))
  4: ggplot_build(x)
  3: print.ggplot(p)
  2: print(p)
  1: print(p)
  </code></pre>