Getting “npc” coordinates of ggplot2 grob

问题

The grid package used for generating graphics in the R language uses so-called "normal parent coordinates" (npc) to position graphical objects ("grobs"). Is it possible to get the npc coordinates of a ggplot2 grob? This would be of major help for drawing lines (for connecting data points) across facets (ggplot, drawing line between points across facets contains a solution, but this is very hard to follow for beginners)

library(ggplot2)
library(grid)

# Generate some sample data
df <- data.frame(y=c(1,2,3),x=1,Set=LETTERS[1:3])

# Plot
ggplot(df,aes(x,y)) + 
  geom_point(aes(fill=Set),color="black",shape=21,size=3) + 
  facet_grid(~Set)

# It is easy to list the grobs, but I can't see their npc coordinates
grid.ls(grid.force())

来源：https://stackoverflow.com/questions/48710478/getting-npc-coordinates-of-ggplot2-grob