dual axis.R

## Putting plots together ##################
# extract gtable
g1 <- ggplot_gtable(ggplot_build(p1))
g2 <- ggplot_gtable(ggplot_build(p2))

# overlap the panel of 2nd plot on that of 1st plot
pp <- c(subset(g1$layout, grepl("panel",name) , se = t:r))
g <- gtable_add_grob(g1, g2$grobs[grep("panel",g2$layout$name)], pp$t, 
                     pp$l, pp$b, pp$l)



# axis tweaks
ia <- which(grepl("axis_l",g2$layout$name) |  grepl("axis-l",g2$layout$name)     )
ga <- g2$grobs[ia]


axis_idx <- as.numeric(which(sapply(ga,function(x) !is.null(x$children$axis))))

for(i in 1:length(axis_idx)){
  ax <- ga[[axis_idx[i]]]$children$axis
  ax$widths <- rev(ax$widths)
  ax$grobs <- rev(ax$grobs)
  ax$grobs[[1]]$x <- ax$grobs[[1]]$x - unit(1, "npc") + unit(0.15, "cm")
  g <- gtable_add_cols(g, g2$widths[g2$layout[ia[axis_idx[i]], ]$l], length(g$widths) - 1)
  g <- gtable_add_grob(g, ax, pp$t[axis_idx[i]], length(g$widths) - i, pp$b[axis_idx[i]])
}



# Plot!
grid.newpage()
grid.draw(g)