fix #6521

Author: teunbrand

R/position-stack.R

@@ -154,7 +154,7 @@ PositionStack <- ggproto("PositionStack", Position,
     flipped_aes <- has_flipped_aes(data)
     data <- flip_data(data, flipped_aes)
     var <- self$var %||% stack_var(data)
-    if (!vec_duplicate_any(data$x)) {
+    if (!vec_duplicate_any(data$x)  && !isTRUE(self$fill)) {
       # We skip stacking when all data have different x positions so that
       # there is nothing to stack
       var <- NULL

tests/testthat/test-position-stack.R

@@ -64,6 +64,12 @@ test_that("position_stack() can stack correctly when ymax is NA", {
   expect_equal(get_layer_data(p)$y, c(1, 2))
 })
 
+test_that("position_fill() handles one group per position case", {
+  df <- data_frame0(x = c("A", "B", "C"), y = 1:3)
+  p <- ggplot(df, aes(x, y)) + geom_col(position = "fill")
+  expect_equal(get_layer_data(p)$y, c(1, 1, 1))
+})
+
 # Visual tests ------------------------------------------------------------
 
 test_that("Stacking produces the expected output", {