mapped_discrete has resolution of 1 (#5247)

* mapped_discrete has resolution 1

* Add resolution test

* Add NEWS bullet

Author: teunbrand

NEWS.md

@@ -1,5 +1,8 @@
 # ggplot2 (development version)
 
+* To improve `width` calculation in bar plots with empty factor levels, 
+  `resolution()` considers `mapped_discrete` values as having resolution 1 
+  (@teunbrand, #5211)
 * When `geom_path()` has aesthetics varying within groups, the `arrow()` is
   applied to groups instead of individual segments (@teunbrand, #4935).
 * The default width of `geom_bar()` is now based on panel-wise resolution of

R/utilities-resolution.R

@@ -18,8 +18,10 @@
 #' resolution(c(2, 10, 20, 50))
 #' resolution(c(2L, 10L, 20L, 50L))
 resolution <- function(x, zero = TRUE) {
-  if (is.integer(x) || zero_range(range(x, na.rm = TRUE)))
+  if (is.integer(x) || is_mapped_discrete(x) ||
+      zero_range(range(x, na.rm = TRUE))) {
     return(1)
+  }
 
   x <- unique0(as.numeric(x))
   if (zero) {

tests/testthat/test-utilities.R

@@ -166,3 +166,11 @@ test_that("vec_rbind0 can combined ordered factors", {
   expect_equal(levels(test$a), c("A", "B", "C"))
 
 })
+
+test_that("resolution() gives correct answers", {
+  expect_equal(resolution(c(4,  6)), 2)
+  expect_equal(resolution(c(4L, 6L)), 1L)
+  expect_equal(resolution(mapped_discrete(c(4, 6))), 1L)
+  expect_equal(resolution(c(0, 0)), 1L)
+  expect_equal(resolution(c(0.5,  1.5), zero = TRUE), 0.5)
+})