Improve error from fortify.grouped_df when dplyr is not installed

Fixes #2822

Author: jimhester

NEWS.md

@@ -1,5 +1,8 @@
 # ggplot2 3.0.0.9000
 
+*   `fortify()` now displays a more informative error message for
+    `grouped_df()` objects when dplyr is not installed (@jimhester, #2822).
+
 *   All `geom_*()` now display an informative error message when required 
     aesthetics are missing (@dpseidel, #2637 and #2706).
 

R/fortify.r

@@ -28,6 +28,9 @@ fortify.NULL <- function(model, data, ...) waiver()
 fortify.function <- function(model, data, ...) model
 #' @export
 fortify.grouped_df <- function(model, data, ...) {
+  if (!requireNamespace("dplyr", quietly = TRUE)) {
+    stop("dplyr must be installed to work with grouped_df objects", call. = FALSE)
+  }
   model$.group <- dplyr::group_indices(model)
   model
 }