Add A Horizontal Line To Plot And Legend In Ggplot2
Answer : (1) Try this: cutoff <- data.frame( x = c(-Inf, Inf), y = 50, cutoff = factor(50) ) ggplot(the.data, aes( year, value ) ) + geom_point(aes( colour = source )) + geom_smooth(aes( group = 1 )) + geom_line(aes( x, y, linetype = cutoff ), cutoff) (2) Regarding your comment, if you don't want the cutoff listed as a separate legend it would be easier to just label the cutoff line right on the plot: ggplot(the.data, aes( year, value ) ) + geom_point(aes( colour = source )) + geom_smooth(aes( group = 1 )) + geom_hline(yintercept = 50) + annotate("text", min(the.data$year), 50, vjust = -1, label = "Cutoff") Update This seems even better and generalizes to mulitple lines as shown: line.data <- data.frame(yintercept = c(50, 60), Lines = c("lower", "upper")) ggplot(the.data, aes( year, value ) ) + geom_point(aes( colour = source )) + geom_smooth(aes( group ...