facet_grid(space = "free") can work with coord_fixed()

NEWS.md

@@ -1,5 +1,7 @@
 # ggplot2 (development version)
 
+* `facet_grid(space = "free")` can now be combined with `coord_fixed()` 
+  (@teunbrand, #4584).
 * `stat_bin()` now accepts functions for argument `breaks` (@aijordan, #4561)
 * (internal) The plot's layout now has a coord parameter that is used to 
   prevent setting up identical panel parameters (#5427)

R/facet-.R

@@ -139,18 +139,22 @@ Facet <- ggproto("Facet", NULL,
     free  <- params$free       %||% list(x = FALSE, y = FALSE)
     space <- params$space_free %||% list(x = FALSE, y = FALSE)
 
-    if ((free$x || free$y) && !coord$is_free()) {
-      cli::cli_abort(
-        "{.fn {snake_class(self)}} can't use free scales with \\
-        {.fn {snake_class(coord)}}."
-      )
-    }
-
     aspect_ratio <- theme$aspect.ratio
     if (!is.null(aspect_ratio) && (space$x || space$y)) {
       cli::cli_abort("Free scales cannot be mixed with a fixed aspect ratio.")
     }
 
+    if (!coord$is_free()) {
+      if (space$x && space$y) {
+        aspect_ratio <- aspect_ratio %||% coord$ratio
+      } else if (free$x || free$y) {
+        cli::cli_abort(
+          "{.fn {snake_class(self)}} can't use free scales with \\
+          {.fn {snake_class(coord)}}."
+        )
+      }
+    }
+
     table <- self$init_gtable(
       panels, layout, theme, ranges, params,
       aspect_ratio = aspect_ratio %||% coord$aspect(ranges[[1]]),
@@ -220,7 +224,7 @@ Facet <- ggproto("Facet", NULL,
     if (space$y) {
       idx <- layout$PANEL[layout$COL == 1]
       heights <- vapply(idx, function(i) diff(ranges[[i]]$y.range), numeric(1))
-      heights <- unit(heights, "null")
+      heights <- unit(heights * abs(aspect_ratio %||% 1), "null")
     }
 
     # Build gtable

tests/testthat/test-facet-layout.R

@@ -232,6 +232,23 @@ test_that("facet_grid throws errors at bad layout specs", {
   expect_snapshot_error(ggplotGrob(p))
 })
 
+test_that("facet_grid can respect coord aspect with free scales/space", {
+  df <- expand.grid(x = letters[1:6], y = LETTERS[1:3])
+  p <- ggplot(df, aes(x, y)) +
+    geom_tile() +
+    facet_grid(
+      rows = vars(y == "C"),
+      cols = vars(x %in% c("e", "f")),
+      scales = "free", space = "free"
+    ) +
+    coord_fixed(3, expand = FALSE)
+  gt <- ggplotGrob(p)
+  width  <- gt$widths[panel_cols(gt)$l]
+  height <- gt$heights[panel_rows(gt)$t]
+  expect_equal(as.numeric(width),  c(4, 2))
+  expect_equal(as.numeric(height), c(6, 3))
+})
+
 test_that("facet_wrap and facet_grid throws errors when using reserved words", {
   mtcars2 <- mtcars
   mtcars2$PANEL <- mtcars2$cyl