From 835cf08e400ce60e05c04c50f9d8ce9d5102b51f Mon Sep 17 00:00:00 2001
From: Teun van den Brand <tahvdbrand@gmail.com>
Date: Fri, 14 Jun 2024 11:46:24 +0200
Subject: [PATCH 1/3] evaluate facets before adding margins

---
 R/facet-grid-.R | 19 ++++++++++++++-----
 1 file changed, 14 insertions(+), 5 deletions(-)

diff --git a/R/facet-grid-.R b/R/facet-grid-.R
index 0854b5299b..7dc9beb971 100644
--- a/R/facet-grid-.R
+++ b/R/facet-grid-.R
@@ -297,12 +297,21 @@ FacetGrid <- ggproto("FacetGrid", Facet,
       return(data)
     }
 
-    # Compute faceting values and add margins
-    margin_vars <- list(intersect(names(rows), names(data)),
-      intersect(names(cols), names(data)))
-    data <- reshape_add_margins(data, margin_vars, params$margins)
-
+    # Compute faceting values
     facet_vals <- eval_facets(c(rows, cols), data, params$.possible_columns)
+    if (nrow(facet_vals) == nrow(data)) {
+      # Margins are computed on evaluated faceting values (#1864).
+      facet_vals <- reshape_add_margins(
+        # We add an index column to track data recycling
+        vec_cbind(facet_vals, .index = seq_len(nrow(facet_vals))),
+        list(intersect(names(rows), names(facet_vals)),
+             intersect(names(cols), names(facet_vals))),
+        params$margins
+      )
+      # Apply recycling on original data to fit margins
+      data <- vec_slice(data, facet_vals$.index)
+      facet_vals$.index <- NULL
+    }
 
     # If any faceting variables are missing, add them in by
     # duplicating the data

From 42e10e9ac2f5a275e125b4ab85d8b22ac39e76f5 Mon Sep 17 00:00:00 2001
From: Teun van den Brand <tahvdbrand@gmail.com>
Date: Fri, 14 Jun 2024 11:46:29 +0200
Subject: [PATCH 2/3] add test

---
 tests/testthat/test-facet-map.R | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/tests/testthat/test-facet-map.R b/tests/testthat/test-facet-map.R
index 35e3256958..de2bf20af2 100644
--- a/tests/testthat/test-facet-map.R
+++ b/tests/testthat/test-facet-map.R
@@ -22,6 +22,10 @@ test_that("margins add extra data", {
   loc <- panel_map_one(facet_grid(a~b, margins = "b"), df)
 
   expect_equal(nrow(loc), nrow(df) * 2)
+
+  # For variables including computation (#1864)
+  loc <- panel_map_one(facet_grid(a ~ I(b + 1), margins = TRUE), df)
+  expect_equal(nrow(loc), nrow(df) * 4)
 })
 
 test_that("grid: missing facet columns are duplicated", {

From 23e2c116fe94562c78bb2d867d70efb8cdd49e18 Mon Sep 17 00:00:00 2001
From: Teun van den Brand <tahvdbrand@gmail.com>
Date: Fri, 14 Jun 2024 11:48:31 +0200
Subject: [PATCH 3/3] add news bullet

---
 NEWS.md | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/NEWS.md b/NEWS.md
index 67c07b0b05..10eea3f3c2 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -1,5 +1,7 @@
 # ggplot2 (development version)
 
+* Fixed bug in `facet_grid(margins = TRUE)` when using expresssions 
+  (@teunbrand, #1864).
 * `geom_rug()` prints a warning when `na.rm = FALSE`, as per documentation (@pn317, #5905)
 * `position_dodge(preserve = "single")` now handles multi-row geoms better,
   such as `geom_violin()` (@teunbrand based on @clauswilke's work, #2801).