Finding a relative axis position on a ggplot-CodePudding

I would like to figure out how to get a y position that is relative to the scale of the ggplot produced.

For example:

library(ggplot)

marks <- data.frame(mpg = c(15,25,30), cyl = c(4,8,4))

ggplot() 
  geom_point(mtcars,mapping = aes(x = mpg, y = hp)) 
  facet_grid(~cyl) 
  geom_point(marks, mapping = aes(x = mpg, y = Inf), col = "red")

Inserting Inf or -Inf will you the maximum and minimum extent of the plot. But what if I want 90%, which in this case would be probably around 330 maybe, however I do not want to hard code it.

CodePudding user response：

One option to achieve your desired result would be via the gggrid package which similar to annotation_custom allows to add your marks as grobs using relative coordinates but works with facets too:

library(ggplot2)

marks <- data.frame(mpg = c(15,25,30), cyl = c(4,8,4))

pg <- function(data, coords) {
  grid::pointsGrob(x = coords$x, y = unit(rep(.9, length(coords$x)), "npc"),
                   gp = grid::gpar(col = "red", size = 1), pch = 16, size = unit(2, "mm"))
}

ggplot() 
  geom_point(mtcars,mapping = aes(x = mpg, y = hp)) 
  facet_grid(~cyl)  
  gggrid::grid_panel(pg, data = marks, mapping = aes(x = mpg))

CodePudding user response：

Once you have created the plot you can get the range using the ggplot_build function to find plot range. Not sure if you can access this without creating the plot as an object first.

library(ggplot)

marks <- data.frame(mpg = c(15,25,30), cyl = c(4,8,4))

p1 <- ggplot() 
  geom_point(mtcars,mapping = aes(x = mpg, y = hp)) 
  facet_wrap(~cyl)

prange <- ggplot_build(p1)$layout$panel_params[[1]]$y$continuous_range

p1   
  geom_point(marks, mapping = aes(x = mpg, y = prange[1]   (diff(prange)*0.9)), col = "red")