ggplot includes built in and seamless functionality that summarises your data before plotting it. As shown in the example below, ggplot_build() can be used to access the summarised dataset.

summarised_barplot

library(ggplot2)
p = ggplot(diamonds, aes(x = color, fill=cut(price,
breaks = quantile(price),
include.lowest=TRUE,
labels = c('Expensive',
'More expensive',
'Very expensive',
'Ridiculously expensive')))) +
geom_bar(position='fill') +
facet_wrap(~cut) +
scale_fill_brewer('Price', palette='OrRd') +
theme(legend.position='top')
p
p_build = ggplot_build(p)
plot_data = as.data.frame(p_build$data)
head(plot_data)
view raw get_ggplot_data.R hosted with ❤ by GitHub 
fill         y count prop x PANEL group    ...
#D7301F 0.2147239    35    1 1     1     4 ...
#FC8D59 0.6871166    77    1 1     1     3 ...
#FDCC8A 0.9570552    44    1 1     1     2 ...
#FEF0D9 1.0000000     7    1 1     1     1 ...
#D7301F 0.1696429    38    1 2     1     8 ...
#FC8D59 0.6116071    99    1 2     1     7 ...
...