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