R: Combining "vline" and "hline" statements together (ggplot2)

Question

I am working with the R programming language. Recently, I learned how to "draw" horizontal and vertical lines with the ggplot library:

library(ggplot2)

# Simple scatter plot
sp <- ggplot(data=mtcars, aes(x=wt, y=mpg)) + geom_point()

# Add horizontal line at y = 2O

a = sp + geom_hline(yintercept=20) 

b = sp + geom_vline(xintercept = 3)

Now, I am trying to combine these statements together - for example:

c = sp + geom_hline(yintercept=20) +  geom_hline(yintercept=15) + geom_vline(xintercept = 3) +
    geom_vline(xintercept = 5) + geom_vline(xintercept = 6) + geom_hline(yintercept=35)

Question: I am trying to modify the above plot so that it looks like this:

Can someone please show me how to do this? Or do you have to manually export this plot into Microsoft Paint and change it over there?

Thanks

Answer 1

You could add a layer using annotate. Each segment is a matched element of the four vectors below. ie the first segment starts at (-Inf,15) and ends at (3,15).

ggplot(data=mtcars, aes(x=wt, y=mpg)) + geom_point() +
  annotate("segment", 
           x = c(-Inf, 3, -Inf, 5, -Inf, 6),
           y = c(15, 15, 20, 20, 35, 35),
           xend = c(3, 3, 5, 5, 6, 6),
           yend = c(15, -Inf, 20, -Inf, 35, -Inf))

Or perhaps it would be easier to define a function which draws two lines from a point toward the axes (assuming here the point is positive so it should head toward -Inf). To make a function output multiple ggplot2 elements, put them in a list:

draw_guides<- function(x, y) {
  list(geom_segment(aes(x = -Inf, xend = x, y = y, yend = y)),
       geom_segment(aes(x = x, xend = x, y = y, yend = -Inf)))
}

Then you can call each one to recreate the desired figure more easily:

ggplot(data=mtcars, aes(x=wt, y=mpg)) + geom_point() +
  draw_guides(3,15) +
  draw_guides(5,20) +
  draw_guides(6,35)