Better computation for missing layers.

Add unit test

Author: hadley

R/geom-.r

@@ -68,7 +68,7 @@ Geom <- ggproto("Geom",
 
   draw_layer = function(self, data, params, panel, coord) {
     if (empty(data)) {
-      n <- nlevels(data$PANEL)
+      n <- if (is.factor(data$PANEL)) nlevels(data$PANEL) else 1L
       return(rep(list(zeroGrob()), n))
     }
 

tests/testthat/test-empty-data.r

@@ -95,3 +95,11 @@ test_that("empty layers still generate one grob per panel", {
 
   expect_equal(length(layer_grob(d)), 3)
 })
+
+test_that("missing layers generate one grob per panel", {
+  df <- data.frame(x = 1:4, y = 1:2, g = 1:2)
+  base <- ggplot(df, aes(x, y)) + geom_point(shape = NA, na.rm = TRUE)
+
+  expect_equal(length(layer_grob(base)), 1)
+  expect_equal(length(layer_grob(base + facet_wrap(~ g))), 2)
+})