title: R ggplot2 color mapping with geom_hline
body: <p>I am trying to map color variable to geom_hline, but doesn't seem to work like other geometries. Neither of these work, in each case, rendering a fixed black line.</p>
  
  <pre><code>ggplot(data.frame(x=0,y=0,series="ABC"),aes(x,y,color=series))) + 
    geom_point() +
    geom_hline(yintercept=0,show.legend = TRUE)
  
  ggplot(data.frame(x=0,y=0),aes(x,y)) + 
    geom_point() +
    geom_hline(yintercept=0,aes(color="ABC"),show.legend = TRUE)
  </code></pre>
  
  <p>Is this a bug, or is there some syntax I am missing?</p>