layer_*() functions default to plot = last_plot() (#5187)

* Set `last_plot()` as default `plot` argument in layer* functions

* Add NEWS bullet

Author: teunbrand

NEWS.md

@@ -1,5 +1,7 @@
 # ggplot2 (development version)
 
+* The `layer_data()`, `layer_scales()` and `layer_grob()` now have the default
+  `plot = last_plot()` (@teunbrand, #5166).
 * To prevent changing the plotting order, `stat_sf()` is now computed per panel 
   instead of per group (@teunbrand, #4340).
 * ggplot2 now uses `scales::DiscreteRange` and `scales::ContinuousRange`, which

R/plot-build.r

@@ -111,13 +111,13 @@ ggplot_build.ggplot <- function(plot) {
 
 #' @export
 #' @rdname ggplot_build
-layer_data <- function(plot, i = 1L) {
+layer_data <- function(plot = last_plot(), i = 1L) {
   ggplot_build(plot)$data[[i]]
 }
 
 #' @export
 #' @rdname ggplot_build
-layer_scales <- function(plot, i = 1L, j = 1L) {
+layer_scales <- function(plot = last_plot(), i = 1L, j = 1L) {
   b <- ggplot_build(plot)
 
   layout <- b$layout$layout
@@ -131,7 +131,7 @@ layer_scales <- function(plot, i = 1L, j = 1L) {
 
 #' @export
 #' @rdname ggplot_build
-layer_grob <- function(plot, i = 1L) {
+layer_grob <- function(plot = last_plot(), i = 1L) {
   b <- ggplot_build(plot)
 
   b$plot$layers[[i]]$draw_geom(b$data[[i]], b$layout)