Only use dplyr conditionally

Needed for CRAN submission

Author: hadley

NAMESPACE

@@ -42,6 +42,7 @@ S3method(fortify,sfc)
 S3method(fortify,sfg)
 S3method(fortify,summary.glht)
 S3method(fortify,tbl)
+S3method(fortify,tbl_df)
 S3method(ggplot,"function")
 S3method(ggplot,default)
 S3method(ggplot_add,"NULL")

R/fortify.r

@@ -14,7 +14,14 @@ fortify <- function(model, data, ...) UseMethod("fortify")
 #' @export
 fortify.data.frame <- function(model, data, ...) model
 #' @export
-fortify.tbl <- function(model, data, ...) dplyr::collect(model)
+fortify.tbl_df <- function(model, data, ...) model
+#' @export
+fortify.tbl <- function(model, data, ...) {
+  if (!requireNamespace("dplyr", quietly = TRUE)) {
+    stop("dplyr must be installed to work with tbl objects", call. = FALSE)
+  }
+  dplyr::collect(model)
+}
 #' @export
 fortify.NULL <- function(model, data, ...) waiver()
 #' @export