How to add a line under ggplot axis text

Question

I want to add a pair of horizontal lines at the bottom of ggplot to illustrate grouping of the x axis elements. It looks like both geom_abline and geom_segment will add lines inside the graph. Any idea how to add lines under it ?

p + geom_segment(aes(x = 2, y = -0.5, xend = 4, yend = -0.5 )) +
        geom_segment(aes(x = 5, y = -0.5, xend = 7, yend = -0.5))

this plots the line segment within the plot not under the axis title.

ggplot alone can only do annotations within the plot area, but several extension packages let you annotate outside the plot area. I know cowplot can do this, there may be other packages that can too — Jan Boyer
Thanks for the information. The answer below is useful, but I will keep your comment in mind and try cowplot. — PDM

phalteman phalteman · Accepted Answer · 2019-04-08T20:01:05

You can make annotations outside the plot area by using coord_cartesian(xlim, ylim, clip="off") and then using annotate() with the appropriate geom. Depending on your aesthetic for grouping, you can put lines at the base of the plot area or below the axis labels.

library(ggplot2)

df <- data.frame(x=seq(1,10), y=seq(1,10))

ggplot(df, aes(x,y)) +
  geom_point() +
  coord_cartesian(xlim=c(0,10), ylim=c(0,10), clip="off") +
  annotate("segment", x = 2, xend = 4, y = -1, yend = -1) +
  annotate("segment", x = 5, xend = 7, y = -0.5, yend = -0.5)

How to add a line under ggplot axis text

1 Answers