Extract data from a ggplot

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To get values actually plotted you can use function ggplot_build() where argument is your plot.

p <- ggplot(mtcars,aes(mpg))+geom_histogram()+
      facet_wrap(~cyl)+geom_vline(data=data.frame(x=c(20,30)),aes(xintercept=x))

pg <- ggplot_build(p)

This will make list and one of sublists is named data. This sublist contains dataframe with values used in plot, for example, for histrogramm it contains y values (the same as count). If you use facets then column PANEL shows in which facet values are used. If there are more than one geom_ in your plot then data will contains dataframes for each – in my example there is one dataframe for histogramm and another for vlines. 

head(pg$data[[1]])
  y count         x ndensity ncount density PANEL group ymin ymax
1 0     0  9.791667        0      0       0     1     1    0    0
2 0     0 10.575000        0      0       0     1     1    0    0
3 0     0 11.358333        0      0       0     1     1    0    0
4 0     0 12.141667        0      0       0     1     1    0    0
5 0     0 12.925000        0      0       0     1     1    0    0
6 0     0 13.708333        0      0       0     1     1    0    0
      xmin     xmax
1  9.40000 10.18333
2 10.18333 10.96667
3 10.96667 11.75000
4 11.75000 12.53333
5 12.53333 13.31667
6 13.31667 14.10000

head(pg$data[[2]])
  xintercept PANEL group xend  x
1         20     1     1   20 20
2         30     1     1   30 30
3         20     2     2   20 20
4         30     2     2   30 30
5         20     3     3   20 20
6         30     3     3   30 30

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