Better error message when geom_boxplot() gets group data with more than 1 row (#3321)

* fix typo in geom_boxplot comment

* better error message when geom_boxplot() gets data with more than 1 row (Fixes #3316)

* Fix typo, Fixes #3316

Author: paleolimbot

R/geom-boxplot.r

@@ -159,7 +159,7 @@ geom_boxplot <- function(mapping = NULL, data = NULL,
 #' @export
 GeomBoxplot <- ggproto("GeomBoxplot", Geom,
 
-  # need to declare `width`` here in case this geom is used with a stat that
+  # need to declare `width` here in case this geom is used with a stat that
   # doesn't have a `width` parameter (e.g., `stat_identity`).
   extra_params = c("na.rm", "width"),
 
@@ -200,6 +200,14 @@ GeomBoxplot <- ggproto("GeomBoxplot", Geom,
                         outlier.alpha = NULL,
                         notch = FALSE, notchwidth = 0.5, varwidth = FALSE) {
 
+    # this may occur when using geom_boxplot(stat = "identity")
+    if (nrow(data) != 1) {
+      stop(
+        "Can't draw more than one boxplot per group. Did you forget aes(group = ...)?",
+        call. = FALSE
+      )
+    }
+
     common <- list(
       colour = data$colour,
       size = data$size,

tests/testthat/test-geom-boxplot.R

@@ -47,6 +47,16 @@ test_that("boxes with variable widths do not overlap", {
   expect_false(any(duplicated(xid)))
 })
 
+test_that("boxplots with a group size >1 error", {
+  p <- ggplot(
+    data_frame(x = "one value", y = 3, value = 4:6),
+    aes(x, ymin = 0, lower = 1, middle = y, upper = value, ymax = 10)
+  ) +
+    geom_boxplot(stat = "identity")
+
+  expect_equal(nrow(layer_data(p, 1)), 3)
+  expect_error(layer_grob(p, 1), "Can't draw more than one boxplot")
+})
 
 # Visual tests ------------------------------------------------------------