accept NULL or NA for datum in st_graticule

related to tidyverse/ggplot2#2200

Author: edzer

R/graticule.R

@@ -13,9 +13,9 @@
 #'  better than a graticule.
 #'
 #' @export
-#' @param x object of class \code{sf}, \code{sfc} or \code{sfg} or numeric vector with bounding box (minx,miny,maxx,maxy).
+#' @param x object of class \code{sf}, \code{sfc} or \code{sfg} or numeric vector with bounding box given as (minx, miny, maxx, maxy).
 #' @param crs object of class \code{crs}, with the display coordinate reference system
-#' @param datum object of class \code{crs}, with the coordinate reference system for the graticules
+#' @param datum either an object of class \code{crs} with the coordinate reference system for the graticules, or \code{NULL} or \code{NA} in which case a grid in the coordinate system of \code{x} is drawn.
 #' @param lon numeric; degrees east for the meridians
 #' @param lat numeric; degrees north for the parallels
 #' @param ndiscr integer; number of points to discretize a parallel or meridian
@@ -73,6 +73,9 @@ st_graticule = function(x = c(-180,-90,180,90), crs = st_crs(x),
 
 	if (is.null(crs))
 		crs = NA_crs_
+	
+	if (is.null(datum) || is.na(datum))
+		datum = crs
 
 	# Get the bounding box of the plotting space, in crs
 	bb = if (inherits(x, "sf") || inherits(x, "sfc") || inherits(x, "sfg"))

man/st_graticule.Rd

@@ -45,6 +45,6 @@ plot(nc[1], graticule = st_crs(nc), axes = TRUE)
 g = st_graticule()
 
 library(ggplot2)
-if (utils::packageVersion("ggplot2") >= "2.2.1") {
+if (utils::packageVersion("ggplot2") > "2.2.1") {
   ggplot() + geom_sf(data = st_set_crs(nc, NA_crs_)) # NA_crs_ for crs
 }

tests/graticule.R

@@ -1,6 +1,6 @@
 
-R version 3.3.2 (2016-10-31) -- "Sincere Pumpkin Patch"
-Copyright (C) 2016 The R Foundation for Statistical Computing
+R version 3.4.1 (2017-06-30) -- "Single Candle"
+Copyright (C) 2017 The R Foundation for Statistical Computing
 Platform: x86_64-pc-linux-gnu (64-bit)
 
 R is free software and comes with ABSOLUTELY NO WARRANTY.
@@ -61,6 +61,11 @@ Type 'q()' to quit R.
 > 
 > g = st_graticule()
 > 
+> library(ggplot2)
+> if (utils::packageVersion("ggplot2") > "2.2.1") {
++   ggplot() + geom_sf(data = st_set_crs(nc, NA_crs_)) # NA_crs_ for crs
++ }
+> 
 > proc.time()
    user  system elapsed 
-  0.972   0.024   0.994 
+  2.212   0.296   2.198