Hugh Hugh - 3 days ago 4
R Question

"Unitless", qualitative, or relative axis scales ggplot2

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

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

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