Editing raw grid objects from a ggplot

Editing raw Grid objects from a ggplot

1. save the plot as a grob
2. locate the particular grob in the gTree
3. call grid.edit() with the gPath constructed from the information of step #2

A simple example goes like this, (adpated from r-help, http://finzi.psych.upenn.edu/Rhelp08/2009-May/199976.html)

  library(ggplot2)
  p <- # minimal example
  qplot(0,0, geom="blank")+ annotate("text",0,0,label="alpha")
  p
 
  g <- # store the plot as a grob
  ggplotGrob(p)
 
  # structure of the grob
  # grid.ls(g) # rather large!
 
  # find a particular grob in the gTree
  # grid.ls(grob=F, view=T)
  grid.ls(getGrob(g,"texts", grep = T))
 
  grid.edit(gPath("texts.gTree", "GRID.text"), grep=TRUE,
              gp=gpar(col="pink"))
 
  grid.edit(gPath("texts.gTree", "GRID.text"), grep=TRUE,
             label=expression(alpha^2), gp=gpar(col="red"))