title: ggplot2 legend with two different geom_point
body: <p>I have the following ggplot graph with circles representing the observed data and the crosses the mean for each treatment : </p>
  
  <pre><code>d &lt;- data.frame(Number = rnorm(12,100,20), 
                  Treatment = rep(c("A","B","C", "D"), each = 3))
  av &lt;- aggregate(d["Number"], d["Treatment"], mean)
  
  ggplot(data = d, aes(y = Number, x = Treatment)) + 
      geom_point(shape = 1, size = 6,  color = "grey50") +
      geom_point(data=av, shape = 4) +
      theme_bw()
  </code></pre>
  
  <p>I would like to add a legend with the exact same symbols on top of the graphs but I'm a bit lost... I use aes to force the creation of legend and then try to modify it with manual scales but the result is not convincing. I would like to have one grey circle of size 6. That sounds also quite complicated for such a basic thing ... There is probably an easyier solution.</p>
  
  <pre><code>ggplot(data = d, aes(y = Number, x = Treatment)) + 
      geom_point(aes(shape = "1", size = "6",  color = "grey50")) +
      geom_point(data=av, aes(shape = "4")) +
      theme_bw() +
      scale_shape_manual(name = "", values = c(1,4), labels = c("observed values", "mean")) +
      scale_size_manual(name = "", values = c(6,1), labels = c("observed values", "mean")) +
      scale_color_manual(name = "", values = c("grey50","black"), 
                         labels = c("observed values", "mean")) +
      theme(legend.position = "top",
            legend.key = element_rect(color = NA))
  </code></pre>
  
  <p><a href="http://imagizer.imageshack.us/v2/320x240q90/842/4pgj.png" rel="nofollow noreferrer">http://imagizer.imageshack.us/v2/320x240q90/842/4pgj.png</a></p>