r - 从分面 ggplot 中提取行数

Question

考虑一个多面的ggplot

plotdf <- data.frame(x = 1:21, 
                     y = 3*(1:21)+4, 
                     z = c(rep(1,3), rep(2,3), rep(3,3), rep(4,3), rep(5,3), rep(6,3), rep(7,3)))

p <- ggplot2::ggplot(plotdf) + 
     ggplot2::geom_point(ggplot2::aes(x=x,y=y)) + 
     ggplot2::facet_wrap(~ z)

如何从 p 中提取行数(或列数)？

p$facet 似乎包含相关信息:

p$facet
    <ggproto object: Class FacetWrap, Facet>
        compute_layout: function
        draw_back: function
        draw_front: function
        draw_labels: function
        draw_panels: function
        finish_data: function
        init_scales: function
        map: function
        map_data: function
        params: list
        render_back: function
        render_front: function
        render_panels: function
        setup_data: function
        setup_params: function
        shrink: TRUE
        train: function
        train_positions: function
        train_scales: function
        super:  <ggproto object: Class FacetWrap, Facet>

但我无法从中提取 nrow 或 ncol 。有什么建议么？

Answer 1

跟进@Aditya 的回答，这是(从 ggplot 3.1.1 开始):

library(magrittr)
gg_facet_nrow_ng <- function(p){
 assertive.types::assert_is_any_of(p, 'ggplot')
 p %>%
   ggplot2::ggplot_build() %>%
   magrittr::extract2('layout') %>% 
   magrittr::extract2('layout') %>%
   magrittr::extract2('ROW') %>%
   unique() %>%
   length()
}