"Unitless", qualitative, or relative axis scales ggplot2

Question

Suppose I have the following chart

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

ggplot(dat, aes(x=x, y=y)) + geom_point()

but I'm not actually interested in the values 2.5, 5, 7.5, 10 on either axis. All I want to mark is something like "Low to High". I know I can mark + xlab("Low to High") but I would prefer "Low" to be at the far left of the axis (under the origin) and "High" to be at the far right (under 10.0) with perhaps an arrow from Low ---> High. I could specify the breaks manually, but that seems too much of a kludge.

Answer 1

Something like this might help,

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

p <- ggplot(dat, aes(x=x, y=y)) + geom_point() +
  scale_x_continuous('', breaks=NULL)+
  scale_y_continuous('', breaks=NULL)

g <- ggplotGrob(p)
library(gtable)
library(grid)

my_axis <- function(low="low", high="high", axis=c("x", "y"), ...){

  axis <- match.arg(axis)

  if(axis == "x"){
  g1 <- textGrob(low, x=unit(0,"npc"), hjust=0)
  g3 <- textGrob(high, x=unit(1,"npc"), hjust=1)
  g2 <- segmentsGrob(grobWidth(g1) + unit(2,"mm"), unit(0.5,"npc"),
               unit(1,"npc") - grobWidth(g3)- unit(2,"mm"), 
               unit(0.5,"npc"), ...)

  } else if(axis == "y"){
    g1 <- textGrob(low, y=unit(0,"npc"), rot=90, hjust=0)
    g3 <- textGrob(high, y=unit(1,"npc"), rot=90, hjust=1)
    g2 <- segmentsGrob(unit(0.5,"npc"),grobHeight(g1) + unit(2,"mm"), 
                       unit(0.5,"npc"),
                       unit(1,"npc") - grobHeight(g3)- unit(2,"mm"), 
                       ...)

  }

  grobTree(g1,g2,g3)
}

g <- gtable_add_grob(g, my_axis(arrow=arrow(length=unit(2,"mm"))), 
                     t=nrow(g)-2, b=nrow(g)-1, l=4)
g <- gtable_add_grob(g, my_axis(axis="y", arrow=arrow(length=unit(2,"mm"))), 
                     t=3, l=1,r=3)
grid.newpage()
grid.draw(g)