Extract number of rows from faceted ggplot

Question

Consider a faceted 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         


        

Answer 1

If you don't define the number of rows or columns manually, this is calculated for you with wrap_dims.

So, in your example:

n_panels <- length(unique(ggplot_build(p)$data[[1]]$PANEL))

wrap_dims(n_panels)

[1] 3 3

To take into account any manual definitions as well, we can look up those parameters too, e.g. ggplot_build(p)$layout$facet$params$nrow gives the number of rows.

A function then to get the number of rows and columns:

get_row_col <- function(p) {
  n <- length(unique(ggplot_build(p)$data[[1]]$PANEL))
  par <- ggplot_build(p)$layout$facet$params
  wrap_dims(n, par$nrow, par$ncol)
}

> get_row_col(p)
[1] 3 3

Answer 2

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

gg_facet_nrow(p)

Answer 3

Following up on @Aditya's answer, this is (as of ggplot 3.1.1):

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()
}