How to add a horizontal line above the histogram using ggplot?

Question

I would like to add a horizontal line above the line. Currently, I can add points using geom_points :

 data <- ddply(diamonds, .(cut, color), summarise, mean_carat = mean(carat)) ggplot(data, aes(color, mean_carat,fill=cut)) + geom_bar(stat="identity", position="dodge") + geom_point(data=data, aes(color, mean_carat, fill=cut)) 

The red arrow in the image below shows what I would like to add:

Thanks!

Answer 1

Try adding something like

 data <- ddply(diamonds, .(cut, color), summarise, mean_carat = mean(carat)) data2 <- ddply(data, ~color, summarise, max_carat = max(mean_carat)) C <- merge(data,data2, by = c('color')) ggplot(data, aes(color, mean_carat,fill=cut)) + geom_bar(stat="identity", position="dodge") + geom_point(data=data, aes(color, mean_carat)) + geom_errorbar(data = C, aes(y=max_carat, ymax=max_carat, ymin=max_carat))