Warn on unsupported geoms in annotate()

NEWS.md

@@ -1,5 +1,8 @@
 # ggplot2 (development version)
 
+* `annotate()` now documents unsupported geoms (`geom_abline()`, `geom_hline()`
+  and `geom_vline()`), and warns when they are requested (@mikmart, #4719)
+
 * `position_stack()` now works fully with `geom_text()` (@thomasp85, #4367)
 
 * `geom_tile()` now correctly recognises missing data in `xmin`, `xmax`, `ymin`,

R/annotation.r

@@ -11,6 +11,12 @@
 #' set. This means that layers created with this function will never
 #' affect the legend.
 #'
+#' @section Unsupported geoms:
+#' Due to their special nature, reference line geoms [geom_abline()],
+#' [geom_hline()] and [geom_vline()] can't be used with [annotate()].
+#' For annotating reference lines, they should be used directly, instead.
+#'
+#' @seealso [geom_abline()] for annotating reference lines.
 #' @param geom name of geom to use for annotation
 #' @param x,y,xmin,ymin,xmax,ymax,xend,yend positioning aesthetics -
 #'   you must specify at least one of these.
@@ -38,6 +44,10 @@ annotate <- function(geom, x = NULL, y = NULL, xmin = NULL, xmax = NULL,
                      ymin = NULL, ymax = NULL, xend = NULL, yend = NULL, ...,
                      na.rm = FALSE) {
 
+  if (geom %in% c("abline", "hline", "vline")) {
+    warn(glue("`annotate()` does not support `geom = \"{geom}\"`. Please use `geom_{geom}()` directly, instead."))
+  }
+
   position <- compact(list(
     x = x, xmin = xmin, xmax = xmax, xend = xend,
     y = y, ymin = ymin, ymax = ymax, yend = yend

tests/testthat/test-annotate.r

@@ -48,3 +48,9 @@ test_that("annotation_* has dummy data assigned and don't inherit aes", {
   expect_false(map$inherit.aes)
   expect_false(raster$inherit.aes)
 })
+
+test_that("unsupported geoms signal a warning (#4719)", {
+  expect_warning(annotate("abline", slope = 1), "not support")
+  expect_warning(annotate("hline", yintercept = 0), "not support")
+  expect_warning(annotate("vline", xintercept = 0), "not support")
+})