double axis hack in ggplot2

doubleaxis.r

library(gtable)
library(ggplot2)
library(plyr)

set.seed(1)
d <- data.frame(x=rep(1:10, 5),
                y=rnorm(50),
                g = gl(5,10))

# example plot
p <- ggplot(d, aes(x,y,colour=g)) +
  geom_line() +
  scale_x_continuous(expand=c(0,0))

g <- ggplotGrob(p)
# extract the axis only, we don't need the rest
gl <- gtable_filter(ggplotGrob(p), "axis-l")
# messing around with the axis...
gl$grobs[[1]][["children"]]["axis"][[1]][["grobs"]][[1]] <- nullGrob()
w <- gl$grobs[[1]][["children"]]["axis"][[1]][["widths"]][[2]] - unit(0.1,  "cm")
gl$grobs[[1]][["children"]]["axis"][[1]][["widths"]][[2]]  <- w
gl$grobs[[1]][["children"]]["axis"][[1]][["widths"]][[1]] <- unit(0,"line")

# add a cell next to the main plot panel, and insert gl there
index <- subset(g$layout, name == "panel")
g <- gtable_add_cols(g, w, pos=index$r)
g <- gtable_add_grob(g, gl, t = index$t, l=index$r+1, 
                     b=index$b, r=index$r+1)
grid.newpage()
grid.draw(g)

same idea here: http://rpubs.com/kohske/dual_axis_in_ggplot2