title: add geom_hline legend without screwing up geom_line legend
body: <p>I'm trying to draw a simple (scree)-plot with some extra <code>geom_hline</code> and <code>geom_vline</code>s thrown in.</p>
  
  <p>Problem is: whenever I so much as add <code>show_guide=TRUE</code> <em>or</em> add some <code>aes()</code> to the <code>geom_xline</code>, I screw up the original legend.</p>
  
  <p>Here's some (ugly) fake data:</p>
  
  <pre><code>exdf &lt;- data.frame(rep(x=1:12, times = 3), rep(x = c("A", "B", "C"), times = 6), rnorm(36), stringsAsFactors = FALSE)
  colnames(exdf) &lt;- c("PC", "variable", "eigenvalue") 
  </code></pre>
  
  <p>And here's my plot:</p>
  
  <pre><code>g &lt;- ggplot(data = exdf, mapping = aes(x = factor(PC), y = eigenvalue))
  g &lt;- g + geom_line(mapping = aes(group = factor(variable), linetype = variable))
  g &lt;- g + geom_vline(xintercept = 7, colour = "green", show_guide = TRUE)
  </code></pre>
  
  <p><img src="https://i.stack.imgur.com/6D7OK.png" alt="enter image description here">
  <img src="https://i.stack.imgur.com/47ji6.png" alt="enter image description here"></p>
  
  <p>How do I add a separate legend for the <code>geom_vline</code> without polluting the other legend?</p>
  
  <p>Can't wrap my head around why one layer's color would change that of another <em>legend</em>.</p>