better error message when trying to add a function to a plot. Closes #3571. (#3594)

Author: clauswilke

NAMESPACE

@@ -45,6 +45,7 @@ S3method(fortify,tbl_df)
 S3method(ggplot,"function")
 S3method(ggplot,default)
 S3method(ggplot_add,"NULL")
+S3method(ggplot_add,"function")
 S3method(ggplot_add,Coord)
 S3method(ggplot_add,Facet)
 S3method(ggplot_add,Layer)

R/plot-construction.r

@@ -100,6 +100,13 @@ ggplot_add.data.frame <- function(object, plot, object_name) {
   plot
 }
 #' @export
+ggplot_add.function <- function(object, plot, object_name) {
+  stop(
+    "Don't know how to add ", object_name, " to a plot. Did you mean ",
+    object_name, "()?", call. = FALSE
+  )
+}
+#' @export
 ggplot_add.theme <- function(object, plot, object_name) {
   plot$theme <- add_theme(plot$theme, object)
   plot