R ggplot2: Labelling a horizontal line on the y axis with a numeric value

Question

I have a horizontal line in a ggplot and I would like to label it's value (7.1) on the y axis.

library(ggplot2)
df <- data.frame(y=c(1:10),x=c(1:10))
h <- 7.1
plot1 <- ggplot(df, aes(x=x,y=y)) + geom_point() 
plot2 <- plot1+ geom_hline(aes(yintercept=h))

Thank you for your help.

Answer 1

It's not clear if you want 7.1 to be part of the y-axis, or if you just want a way to label the line. Assuming the former, you can use scale_y_continuous() to define your own breaks. Something like this may do what you want (will need some fiddling most likely):

plot1+ geom_hline(aes(yintercept=h)) + 
  scale_y_continuous(breaks = sort(c(seq(min(df$y), max(df$y), length.out=5), h)))

Assuming the latter, this is probably more what you want:

plot1 + geom_hline(aes(yintercept=h)) +
  geom_text(aes(0,h,label = h, vjust = -1))