title: Join with a line the results from stat_summary()
body: <p>I'm trying to make a ggplot graph wich show the proportions of a variable (SIZE) according  to another one (ZONE) using <code>face_grid</code>, then in every category show the mean of a binary variable (BG) using <code>stat_summary</code>. But now, I want to join the points (the means) and show the mean in percent format (near the point). For join the points I've tried with <code>stat_summary('arguments here',geom="line")</code> and other commands but the truth is I don't know how to do it.</p>
  
  <pre><code>    library(ggplot2)
      library(scales)
  
      set.seed(100)
  
      data &lt;- data.frame(ID = 1:600,
                     SIZE = rep(c('B','M','S'), each = 200), 
                     ZONE = sample(c('A','B'), size = 600, replace=T),
                     BG = c(sample(0:1, size = 200, prob = c(1,1), replace=T),
                            sample(0:1, size = 200, prob = c(2,3), replace=T),
                            sample(0:1, size = 200, prob = c(1,2), replace=T)))
  
      ggplot(data, aes(x = SIZE)) + 
        geom_bar(aes(y = (..count..)/sum(..count..))) +
        facet_grid(~ZONE) + 
        stat_summary(aes(y = BG), fun.y=mean, colour="red", geom="point", size = 3) + 
        scale_y_continuous('Percent', labels = percent_format())
  </code></pre>
  
  <p>Thanks in advance and sorry for my english.</p>