title: geom_line connecting the mean of different box plot groups (position identity)
body: <p>I want to draw a geom_line that and connects the mean of each single box plot for the within the different groups (side). Please see my code below </p>
  
  <pre><code>df&lt;-data.frame(time=as.factor(c(1,1,1,1,1,1,2,2,2,2,2,2,3,3,3,3,3,3)), 
                 value=as.numeric(c(12, 8,17,13,12, 9, 10, 12, 9, 11, 10.5, 11.4, 10.9,15,7, 11.6, 12, 13)), 
                 side=as.factor(c("E","Z","E","Z","E","Z","E","Z","E","Z","E","Z","E","Z","E","Z","E","Z")))
  
  df2 &lt;- ddply(df,.(time,side),summarise, val = mean(value))
  
  ggplot(df, aes(x=factor(time), y=value, colour=side))+
  geom_point(position="identity",aes(group=side), colour="black")+
  geom_boxplot(color="black", position = "identity", aes(fill=side))+
  scale_fill_manual(values=c("#999999", "#E69F00"))+
    geom_line(data = df2, aes(y = val, group = val)) 
  </code></pre>