better error message when geom_boxplot() gets data with more than 1 r…

…ow (Fixes tidyverse#3316)

R/geom-boxplot.r

@@ -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 ------------------------------------------------------------