Function to plot secondary axis in ggplot2

2nd_axis.r

library(ggplot2)
library(gtable)

library(grid)
library(ggthemes)

# extract gtable

ggplot_second_axis <- function(p1, p2){
  p2 <- p2 + theme() %+replace% 
    theme(panel.background = element_rect(fill = NA))
  
  
  
  grid.newpage()
  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$layout, name == "panel", se = t:r))
  g <- gtable_add_grob(g1, g2$grobs[[which(g2$layout$name == "panel")]], pp$t, 
                       pp$l, pp$b, pp$l)
  
  # axis tweaks
  ia <- which(g2$layout$name == "axis-l")
  ga <- g2$grobs[[ia]]
  ax <- ga$children[[2]]
  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, ]$l], length(g$widths) - 1)
  g <- gtable_add_grob(g, ax, pp$t, length(g$widths) - 1, pp$b)
  
  ia2 <- which(g2$layout$name == "ylab")
  ga2 <- g2$grobs[[ia2]]
  ga2$rot <- 90
  g <- gtable_add_cols(g, g2$widths[g2$layout[ia2, ]$l], length(g$widths) - 1)
  g <- gtable_add_grob(g, ga2, pp$t, length(g$widths) - 1, pp$b)
  
  return(g)
}



# two plots
p1 <- ggplot(mtcars, aes(mpg, disp)) + geom_line(colour = "blue") 
p2 <- ggplot(mtcars, aes(mpg, drat)) + geom_line(colour = "red") 
f <- ggplot_second_axis(p1,p2)
grid.draw(f)

I've the same problem. How to solve it?
Many thanks

There is a small error in line 34.
change
'ia2 <- which(g2$layout$name == "ylab")'
to
'ia2 <- which(g2$layout$name == "ylab-l")'

You should get the right plot.