NA in manual scale with named values

NEWS.md

@@ -1,5 +1,11 @@
 # ggplot2 (development version)
 
+* Legends in `scale_*_manual()` can show `NA` values again when the `values` is
+  a named vector (@teunbrand, #5214, #5286).
+
+* `scale_*_manual()` with a named `values` argument now emits a warning when
+  none of those names match the values found in the data (@teunbrand, #5298).
+
 * `geom_text()` and `geom_label()` gained a `size.unit` parameter that set the 
   text size to millimetres, points, centimetres, inches or picas 
   (@teunbrand, #3799).

R/scale-manual.R

@@ -141,7 +141,8 @@ scale_discrete_manual <- function(aesthetics, ..., values, breaks = waiver()) {
   manual_scale(aesthetics, values, breaks, ...)
 }
 
-manual_scale <- function(aesthetic, values = NULL, breaks = waiver(), ..., limits = NULL) {
+manual_scale <- function(aesthetic, values = NULL, breaks = waiver(), ...,
+                         limits = NULL) {
   # check for missing `values` parameter, in lieu of providing
   # a default to all the different scale_*_manual() functions
   if (is_missing(values)) {
@@ -152,7 +153,17 @@ manual_scale <- function(aesthetic, values = NULL, breaks = waiver(), ..., limit
 
   if (is.null(limits) && !is.null(names(values))) {
     # Limits as function to access `values` names later on (#4619)
-    limits <- function(x) intersect(x, names(values)) %||% character()
+    force(aesthetic)
+    limits <- function(x) {
+      x <- intersect(x, c(names(values), NA)) %||% character()
+      if (length(x) < 1) {
+        cli::cli_warn(paste0(
+          "No shared levels found between {.code names(values)} of the manual ",
+          "scale and the data's {.field {aesthetic}} values."
+        ))
+      }
+      x
+    }
   }
 
   # order values according to breaks

tests/testthat/_snaps/scale-manual.md

@@ -0,0 +1,4 @@
+# names of values used in manual scales
+
+    No shared levels found between `names(values)` of the manual scale and the data's colour values.
+

tests/testthat/test-scale-manual.R

@@ -6,6 +6,19 @@ test_that("names of values used in manual scales", {
    s2 <- scale_colour_manual(values = c("8" = "c", "4" = "a", "6" = "b"), na.value = NA)
    s2$train(c("4", "8"))
    expect_equal(s2$map(c("4", "6", "8")), c("a", NA, "c"))
+   expect_equal(s2$get_limits(), c("4", "8"))
+
+   s3 <- scale_colour_manual(values = c("8" = "c", "4" = "a", "6" = "b"), na.value = "x")
+   s3$train(c("4", "8", NA))
+   expect_equal(s3$map(c("4", "6", "8")), c("a", "x", "c"))
+   expect_equal(s3$get_limits(), c("4", "8", NA))
+
+   # Names do not match data
+   s <- scale_colour_manual(values = c("foo" = "x", "bar" = "y"))
+   s$train(c("A", "B"))
+   expect_snapshot_warning(
+     expect_equal(s$get_limits(), character())
+   )
 })