How to add a horizontal line above a bar chart using ggplot?

Question

I would like to add a horizontal line above a bar plot. Currently I am able to add dots 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!