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Using grconvertX/grconvertY in ggplot2

I am trying to figure out how to use grconvertX/grconvertX in ggplot. My ultimate goal is to to add annotation to a ggplot2 figure (and possibly lattice) with grid.text and grid.lines by going from user coordinates to device coordinates. I know it can be done with grobs but I am wondering if there is an easier way.

The following code allows me to pass values from user coordinates to ndc coordinates and use those values to annotate the plot with grid.text.

graphics.off()      # close graphics windows   
 
library(grid)
library(gridBase)
 
 
test= data.frame(
  x = c(1,2,3),
  y = c(12,10,3),
  n = c(75,76,73)
  )
 
par(mar = c(13,5,2,3))
 
plot(test$y ~ test$x,type="b", ann=F)
 
for (i in 1:nrow(test))
 
{
  X=grconvertX(i , from="user", to="ndc")
  grid.text(x=X, y =0.2, label=paste("GRID.text at\nuser.x=", i, "\n", "ndc.x=", (signif( X, 5))   ) ) 
  grid.lines(x=c(X, X), y = c(0.28, 0.33) )
}
#add some code to save as PDF ...

The code is based on the solution from one of my previous posts: Mixing X and Y coordinate systems . You can see how x coordinates from the original plot were converted to ndc. The advantage of this approach is that I can use device coordinates for Y.

I assumed I could easily do the same in ggplot2 (and possibly in lattice).

library(ggplot2)
graphics.off()      # close graphics windows   
 
qplot(x=x, y=y, data=test)+geom_line()+  opts(plot.margin = unit(c(1,3,8,1), "lines"))
 
for (i in 1:nrow(test))
 
{
  X=grconvertX(i , from="user", to="ndc")
  grid.text(x=X, y =0.2, label=paste("GRID.text at\nuser.x=", i, "\n", "ndc.x=", (signif( X, 5))   ) ) 
  grid.lines(x=c(X, X), y = c(0.28, 0.33) )
}
 
#add some code to save as PDF...

However, it does not work correctly. The coordinates seem to be a bit off. The vertical lines and text don't correspond to the tick labels on the plot. Can anybody tell me how to fix it? Thanks a lot in advance.