Specify position of geom_text by keywords like “top”, “bottom”, “left”, “right”, “center”

Question

I wish to position text in a ggplot without specifying x and y positions, but instead using keywords, like e.g. in graphics::leg

Answer 1

It's certainly possible to write a wrapper but the way units and justification are defined makes it rather verbose,

library(ggplot2)

qplot(1,1) + 
  annotation_compass('testN') + 
  annotation_compass('testE','E') + 
  annotation_compass('testSW','SW') + 
  annotation_compass('testW','W')

annotation_compass <- function(label,
                               position = c('N','NE','E','SE','S','SW','W','NW'),
                               padding = grid::unit(c(0.5,0.5),"line"), ...){
  position <- match.arg(position)
  x <- switch (position,
    N = 0.5,
    NE = 1,
    E = 1,
    SE = 1,
    S = 0.5, 
    SW = 0,
    W = 0, 
    NW = 0
  )
  y <- switch (position,
               N = 1,
               NE = 1,
               E = 0.5,
               SE = 0,
               S = 0, 
               SW = 0,
               W = 0.5, 
               NW = 1
  )
  hjust <- switch (position,
               N = 0.5,
               NE = 1,
               E = 1,
               SE = 1,
               S = 0.5, 
               SW = 0,
               W = 0, 
               NW = 0
  )
  vjust <- switch (position,
               N = 1,
               NE = 1,
               E = 0.5,
               SE = 0,
               S = 0, 
               SW = 0,
               W = 0.5, 
               NW = 1
  )
  f1 <- switch (position,
                   N = 0,
                   NE = -1,
                   E = -1,
                   SE = -1,
                   S = 0, 
                   SW = 1,
                   W = 1, 
                   NW = 1
  )
  f2 <- switch (position,
                   N = -1,
                   NE = -1,
                   E = 0,
                   SE = 1,
                   S = 1, 
                   SW = 1,
                   W = 0, 
                   NW = -1
  )
  annotation_custom(grid::textGrob(label, 
                                   x=grid::unit(x,"npc") + f1*padding[1] , 
                                   y=grid::unit(y,"npc") + f2*padding[2],
                                   hjust=hjust,vjust=vjust, ...))
}