How to add a label to the x / y axis whenever a vertical / horizontal line is added to a ggplot?

The x and y axis are labeled based on certain interval set by ggplot.

In the event that a horizontal or vertical line is added to the plot, the goal is to label x or y axis at the exact value of the line.

How can this be done?

>Solution :

This isn’t totally straightforward, but it is possible. I would probably use these two little functions to automate the tricky parts:

add_x_break <- function(plot, xval) {
  
  p2 <- ggplot_build(plot)
  breaks <- p2$layout$panel_params[[1]]$x$breaks
  breaks <- breaks[!is.na(breaks)]
  
  plot +
    geom_vline(xintercept = xval) +
    scale_x_continuous(breaks = sort(c(xval, breaks)))
}

add_y_break <- function(plot, yval) {
  
  p2 <- ggplot_build(plot)
  breaks <- p2$layout$panel_params[[1]]$y$breaks
  breaks <- breaks[!is.na(breaks)]
  
  plot +
    geom_hline(yintercept = yval) +
    scale_y_continuous(breaks = sort(c(yval, breaks)))
}

These would work like this. Start with a basic plot:

library(ggplot2)

set.seed(1)

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

p <- ggplot(df, aes(x, y)) + 
  geom_point() +
  ylim(0, 1)

p

Add a vertical line with add_x_break

p <- add_x_break(p, 6.34)

p

Add a horizontal line with add_y_break:

p <- add_y_break(p, 0.333)
#> Scale for 'y' is already present. Adding another scale for 'y', which will
#> replace the existing scale.

p

^{Created on 2022-03-03 by the reprex package (v2.0.1)}