title: Adding points to GGPLOT2 Histogram
body: <p>I'm trying to produce a histogram that illustrates observed points(a sub-set) on a histogram of all observations. To make it meaningful, I need to color each point differently and place a legend on the plot. My problem is, I can't seem to get a scale to show up on the plot. Below is an example of what I've tried.</p>
  
  <pre><code>subset &lt;-1:8
  results = data.frame(x_data = rnorm(5000),TestID=1:5000)
  m &lt;- ggplot(results,aes(x=x_data))
  m+stat_bin(aes(y=..density..))+
  stat_density(colour="blue", fill=NA)+
    geom_point(data = results[results$TestID %in% subset,], 
         aes(x = x_data, y = 0),  
         colour = as.factor(results$TestID[results$TestID %in% subset]),
         size = 5)+
  scale_colour_brewer(type="seq", palette=3)
  </code></pre>
  
  <p>Ideally, I'd like the points to be positioned on the density line(but I'm really unsure of how to make that work, so I'll settle to position them at y = 0). What I need most urgently is a legend which indicates the TestID that corresponds to each of the points in subset.</p>
  
  <p>Thanks a lot to anyone who can help.</p>