Empty layers need to generate grob per layer.

Also add layer_grob() helper. Fixes #1402

Author: hadley

NAMESPACE

@@ -330,6 +330,7 @@ export(labs)
 export(last_plot)
 export(layer)
 export(layer_data)
+export(layer_grob)
 export(lims)
 export(map_data)
 export(margin)

R/layer.r

@@ -217,7 +217,10 @@ Layer <- ggproto("Layer", NULL,
   },
 
   draw_geom = function(self, data, panel, coord) {
-    if (empty(data)) return(list(zeroGrob()))
+    if (empty(data)) {
+      n <- nrow(panel$layout)
+      return(rep(list(zeroGrob()), n))
+    }
 
     data <- self$geom$handle_na(data, self$geom_params)
     self$geom$draw_layer(data, self$geom_params, panel, coord)

R/plot-build.r

@@ -4,7 +4,8 @@
 #' to produce an object that can be rendered.  This function outputs two pieces:
 #' a list of data frames (one for each layer), and a panel object, which
 #' contain all information about axis limits, breaks etc. \code{layer_data}
-#' is a helper function which returns the data for a given layer.
+#' and \code{layer_grob} are helper functions which returns the data or grob
+#' associated with a given layer.
 #'
 #' @param plot ggplot object
 #' @seealso \code{\link{print.ggplot}} and \code{\link{benchplot}} for
@@ -94,6 +95,14 @@ layer_data <- function(plot, i = 1L) {
   ggplot_build(plot)$data[[i]]
 }
 
+#' @export
+#' @rdname ggplot_build
+layer_grob <- function(plot, i = 1L) {
+  b <- ggplot_build(plot)
+
+  b$plot$layers[[i]]$draw_geom(b$data[[i]], b$panel, b$plot$coordinates)
+}
+
 #' Build a plot with all the usual bits and pieces.
 #'
 #' This function builds all grobs necessary for displaying the plot, and

man/ggplot_build.Rd

@@ -84,3 +84,14 @@ test_that("layer inherits data from plot when data = NULL", {
     geom_point(data = NULL)
   expect_equal(nrow(layer_data(d)), nrow(mtcars))
 })
+
+test_that("empty layers still generate one grob per panel", {
+  df <- data.frame(x = 1:3, y = c("a", "b", "c"))
+
+  d <- ggplot(df, aes(x, y)) +
+    geom_point(data = df[0, ]) +
+    geom_point() +
+    facet_wrap(~y)
+
+  expect_equal(length(layer_grob(d)), 3)
+})