Excessive position guide training/drawing in facets

[teunbrand]

When using facets, position guides are trained for every panel and drawn for every row/column. This seems excessive to me.
In the code below, we can count how often a guide is trained and drawn by using the following modification. I hasten to note that this doesn't reflect the intended syntax of extending guides, but it is a quick and dirty hack.

devtools::load_all("~/packages/ggplot2")
#> ℹ Loading ggplot2

train <- 0
draw  <- 0

# Setting up a counting axis
x <- guide_axis()
x$train <- function(self, params, scale, aesthetic = NULL, ...) {
  train <<- train + 1
  GuideAxis$train(params, scale, aesthetic, ...)
}
x$draw <- function(self, theme, params) {
  draw <<- draw + 1
  GuideAxis$draw(theme, params)
}

ggplot(mpg, aes(displ, hwy)) +
  geom_point() +
  guides(x = x) +
  facet_grid(drv ~ cyl)

train
#> [1] 12
draw
#> [1] 4

^{Created on 2023-09-18 with reprex v2.0.2}

Surely, we only need to train or draw* an x-axis just once in the plot above, since all panels share the same scales.
* build the grob once, then re-use for every column (not literally drawing just 1 axis).
Unless there is a very good reason to build more guides than needed, I suppose we could make this more efficient.

[teunbrand]

Similar issue upstream with viewscales:

devtools::load_all("~/packages/ggplot2")
#> ℹ Loading ggplot2
#> Warning: package 'testthat' was built under R version 4.3.1

viewscales <- 0

# Setting up a counting coord
coord <- coord_cartesian()
coord$setup_panel_params <- function(self, scale_x, scale_y, params) {
  viewscales <<- viewscales + 1
  ggproto_parent(CoordCartesian, self)$setup_panel_params(scale_x, scale_y, params)
}

ggplot(mpg, aes(displ, hwy)) +
  geom_point() +
  coord +
  facet_grid(drv ~ cyl)

viewscales
#> [1] 12

^{Created on 2023-09-18 with reprex v2.0.2}