Improve error when piping plots into facets (#4385)

Resolves #4379

Author: mitchelloharawild

NEWS.md

@@ -28,6 +28,9 @@
   installed, which will offer to install the package before continuing (#4375, 
   @malcolmbarrett)
 
+* Improved error with hint when piping a `ggplot` object into a facet function
+  (#4379, @mitchelloharawild).
+
 # ggplot2 3.3.3
 This is a small patch release mainly intended to address changes in R and CRAN.
 It further changes the licensing model of ggplot2 to an MIT license.

R/facet-.r

@@ -276,9 +276,7 @@ df.grid <- function(a, b) {
 # facetting variables.
 
 as_facets_list <- function(x) {
-  if (inherits(x, "uneval")) {
-    abort("Please use `vars()` to supply facet variables")
-  }
+  x <- validate_facets(x)
   if (is_quosures(x)) {
     x <- quos_auto_name(x)
     return(list(x))
@@ -315,6 +313,19 @@ as_facets_list <- function(x) {
   x
 }
 
+validate_facets <- function(x) {
+  if (inherits(x, "uneval")) {
+    abort("Please use `vars()` to supply facet variables")
+  }
+  if (inherits(x, "ggplot")) {
+    abort(
+      "Please use `vars()` to supply facet variables\nDid you use %>% instead of +?"
+    )
+  }
+  x
+}
+
+
 # Flatten a list of quosures objects to a quosures object, and compact it
 compact_facets <- function(x) {
   x <- flatten_if(x, is_list)

R/facet-grid-.r

@@ -157,7 +157,14 @@ grid_as_facets_list <- function(rows, cols) {
   is_rows_vars <- is.null(rows) || is_quosures(rows)
   if (!is_rows_vars) {
     if (!is.null(cols)) {
-      abort("`rows` must be `NULL` or a `vars()` list if `cols` is a `vars()` list")
+      msg <- "`rows` must be `NULL` or a `vars()` list if `cols` is a `vars()` list"
+      if(inherits(rows, "ggplot")) {
+        msg <- paste0(
+          msg, "\n",
+          "Did you use %>% instead of +?"
+        )
+      }
+      abort(msg)
     }
     # For backward-compatibility
     facets_list <- as_facets_list(rows)

R/facet-wrap.r

@@ -86,6 +86,13 @@ facet_wrap <- function(facets, nrow = NULL, ncol = NULL, scales = "fixed",
     x = any(scales %in% c("free_x", "free")),
     y = any(scales %in% c("free_y", "free"))
   )
+
+  # Check for deprecated labellers
+  labeller <- check_labeller(labeller)
+
+  # Flatten all facets dimensions into a single one
+  facets <- wrap_as_facets_list(facets)
+
   if (!is.null(switch)) {
     .Deprecated("strip.position", old = "switch")
     strip.position <- if (switch == "x") "bottom" else "left"
@@ -102,12 +109,6 @@ facet_wrap <- function(facets, nrow = NULL, ncol = NULL, scales = "fixed",
     ncol <- sanitise_dim(ncol)
   }
 
-  # Check for deprecated labellers
-  labeller <- check_labeller(labeller)
-
-  # Flatten all facets dimensions into a single one
-  facets <- wrap_as_facets_list(facets)
-
   ggproto(NULL, FacetWrap,
     shrink = shrink,
     params = list(