facet_wrap(drop = TRUE) can handle character variables with NAs (#5847)

* `ulevels()` only drops character NAs on request

* add test

* add news bullet

Author: teunbrand

NEWS.md

@@ -1,5 +1,9 @@
 # ggplot2 (development version)
 
+* Prevented `facet_wrap(..., drop = FALSE)` from throwing spurious errors when
+  a character facetting variable contained `NA`s (@teunbrand, #5485).
+* When facets coerce the faceting variables to factors, the 'ordered' class
+  is dropped (@teunbrand, #5666).
 * `geom_curve()` now appropriately removes missing data instead of throwing
   errors (@teunbrand, #5831).
 * `update_geom_defaults()` and `update_stat_defaults()` have a reset mechanism

R/facet-grid-.R

@@ -511,11 +511,11 @@ FacetGrid <- ggproto("FacetGrid", Facet,
 
 # Helpers -----------------------------------------------------------------
 
-ulevels <- function(x) {
+ulevels <- function(x, na.last = TRUE) {
   if (is.factor(x)) {
     x <- addNA(x, TRUE)
     factor(levels(x), levels(x), exclude = NULL)
   } else {
-    sort(unique0(x))
+    sort(unique0(x), na.last = na.last)
   }
 }

R/stat-summary-2d.R

@@ -126,7 +126,7 @@ StatSummary2d <- ggproto("StatSummary2d", Stat,
 
 # Adaptation of tapply that returns a data frame instead of a matrix
 tapply_df <- function(x, index, fun, ..., drop = TRUE) {
-  labels <- lapply(index, ulevels)
+  labels <- lapply(index, ulevels, na.last = NA) # drop NA
   out <- expand.grid(labels, KEEP.OUT.ATTRS = FALSE, stringsAsFactors = FALSE)
 
   grps <- split(x, index)

tests/testthat/test-facet-layout.R

@@ -100,7 +100,8 @@ test_that("grid: as.table reverses rows", {
 
 a2 <- data_frame(
   a = factor(1:3, levels = 1:4),
-  b = factor(1:3, levels = 4:1)
+  b = factor(1:3, levels = 4:1),
+  c = as.character(c(1:2, NA))
 )
 
 test_that("wrap: drop = FALSE preserves unused levels", {
@@ -111,6 +112,11 @@ test_that("wrap: drop = FALSE preserves unused levels", {
   wrap_b <- panel_layout(facet_wrap(~b, drop = FALSE), list(a2))
   expect_equal(nrow(wrap_b), 4)
   expect_equal(as.character(wrap_b$b), as.character(4:1))
+
+  # NA character should not be dropped or throw errors #5485
+  wrap_c <- panel_layout(facet_wrap(~c, drop = FALSE), list(a2))
+  expect_equal(nrow(wrap_c), 3)
+  expect_equal(wrap_c$c, a2$c)
 })
 
 test_that("grid: drop = FALSE preserves unused levels", {