"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.

baptiste · Accepted Answer · 2014-10-08T23:50:40.783

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)

enter image description here

+1, but I have to manually specify the horizontal position in `textGrob`. I'd prefer a solution where a change in the values `dat` didn't require other changes. — Hugh, Oct 08 '14 at 23:34
those positions are actually in "npc" units, aka "normalised parent coordinates", aka what you want, I think. — baptiste, Oct 08 '14 at 23:35

score 0 · Answer 2 · answered Dec 01 '16 at 21:27

As an alternative, and since you did ask for "Low ---> High", here's an option that doesn't involve disassembling the plot

dat <- data.frame(x = 1:10, y = 1:10)
low_to_high <- paste0("low   ", paste0(rep("-", 35), collapse = ""), ">   high")

library(ggplot2)
ggplot(dat, aes(x, y)) + 
  geom_point() +
  labs(x = low_to_high, y = low_to_high) +
  coord_equal() +
  theme_bw() +
  theme(axis.title = element_text(size=20), 
        axis.text = element_blank(),
        axis.ticks = element_blank())

Admittedly, the number of - required will vary and needs to be adjusted per your plot size, and it's not as pretty as the properly rendered arrow. Still, this is quick to make from scratch.

"Unitless", qualitative, or relative axis scales ggplot2

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