dismantling a ggplot with grid and gtable

Question

I\'m struggling to build a dual-axis plot based on ggplot objects. At baptiste\'s suggestion, I have broken down the problem into smaller parts. The present iss

Answer 1

A more natural strategy would be to use invisible geom_blank layers, so that ggplot2 still trains the scales etc to build the plot but shows no data. Since you want to process already-formatted plots, however, you probably have to remove manually those grobs from the plot gTree. Here's an attempt,

library(gtable)
g <- ggplotGrob(p)

stripdata <- function(g){
  keep <- grepl("border|grill", 
                names(g[["grobs"]][[4]][["children"]]))
  g[["grobs"]][[4]][["children"]][!keep] <- NULL
  g
}

grid.newpage()
grid.draw(stripdata(g))