Scales with no matching data throw warnings

NEWS.md

@@ -1,5 +1,7 @@
 # ggplot2 (development version)
 
+* Scales now throw warnings when no matching data is detected 
+  (@teunbrand, #3743)
 * The `arrow.fill` parameter is now applied to more line-based functions: 
   `geom_path()`, `geom_line()`, `geom_step()` `geom_function()`, line 
    geometries in `geom_sf()` and `element_line()`.

R/plot-build.R

@@ -102,7 +102,7 @@ ggplot_build.ggplot <- function(plot) {
   # Train and map non-position scales and guides
   npscales <- scales$non_position_scales()
   if (npscales$n() > 0) {
-    lapply(data, npscales$train_df)
+    npscales$train_data(data)
     plot$guides <- plot$guides$build(npscales, plot$layers, plot$labels, data)
     data <- lapply(data, npscales$map_df)
   } else {

R/scales-.R

@@ -68,6 +68,20 @@ ScalesList <- ggproto("ScalesList", NULL,
     lapply(self$scales, function(scale) scale$train_df(df = df))
   },
 
+  train_data = function(self, data) {
+    lapply(data, self$train_df)
+    data_names <- unique0(unlist(lapply(data, colnames)))
+    for (scale in self$scales) {
+      if (!any(scale$aesthetics %in% data_names)) {
+        cli::cli_warn(
+          "Scale for {.field {scale$aesthetics}} aesthetic{?s} was provided \\
+          but has no matching data to map.",
+          call = scale$call
+        )
+      }
+    }
+  },
+
   map_df = function(self, df) {
     if (empty(df) || length(self$scales) == 0) {
       return(df)

tests/testthat/test-scales.R

@@ -71,6 +71,15 @@ test_that("identity scale preserves input values", {
   expect_equal(d1, d2)
 })
 
+test_that("a warning is thrown when a scale is not used", {
+  p <- ggplot(mtcars, aes(disp, mpg)) + geom_point() + scale_colour_discrete()
+
+  expect_warning(
+    ggplot_build(p),
+    "was provided but has no matching data to map"
+  )
+})
+
 test_that("position scales are updated by all position aesthetics", {
   df <- data_frame(x = 1:3, y = 1:3)