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.

get_ggplot_data.R

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)