theme_arrow.r

library(ggplot2)
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)
}


element_grob.custom_axis <- function(element, ...)  {
  my_axis(axis=element$axis,arrow=arrow(length=unit(2,"mm")))
}

## silly wrapper to fool ggplot2
axis_custom <- function(axis = "x"){
  structure(
    list(axis = axis),
    class = c("custom_axis","element_blank", "element") # inheritance test workaround
  ) 
  
}

theme_arrow <- function (base_size = 11, base_family = "") 
{
  theme_grey(base_size = base_size, base_family = base_family) %+replace% 
    theme(axis.line.x = axis_custom(axis="x"),
          axis.line.y=axis_custom(axis="y"),
          axis.title = element_blank(),
          axis.ticks = element_blank(),
          axis.text = element_blank(),
          axis.ticks.length = unit(1,"line"))
}



ggplot(iris, aes(Sepal.Length, Sepal.Width)) +
  facet_wrap(~Species)+
  geom_line() +
  theme_arrow()