Support reverse stacking

Fixes #1837

I also changed the default order to stack from inside-out, rather than bottom-to-top.

NEWS.md

@@ -10,7 +10,7 @@ The main plot title is now left-aligned to better work better with a subtitle. T
 
 ### Stacking
 
-`position_stack()` and `position_fill()` now sort the stacking order to match the order of grouping. This allows you to control the order through grouping, and ensures that the default legend matches the plot (#1552, #1593).
+`position_stack()` and `position_fill()` now sort the stacking order to match the order of grouping. This allows you to control the order through grouping, and ensures that the default legend matches the plot (#1552, #1593). If you want the opposite order (useful if you have horizontal bars and horizontal legend), you can request reverse stacking by using `position = position_stack(reverse = TRUE)` (#1837).
 
 `position_stack()` now accepts negative values which will create stacks extending below the x-axis (#1691).
 

R/position-collide.r

@@ -1,6 +1,6 @@
 # Detect and prevent collisions.
 # Powers dodging, stacking and filling.
-collide <- function(data, width = NULL, name, strategy, ..., check.width = TRUE) {
+collide <- function(data, width = NULL, name, strategy, ..., check.width = TRUE, reverse = FALSE) {
   # Determine width
   if (!is.null(width)) {
     # Width set manually
@@ -26,9 +26,14 @@ collide <- function(data, width = NULL, name, strategy, ..., check.width = TRUE)
     width <- widths[1]
   }
 
-  # Reorder by x position, then on group. Group is reversed so stacking order
-  # follows the default legend order
-  data <- data[order(data$xmin, -data$group), ]
+  # Reorder by x position, then on group. The default stacking order reverses
+  # the group in order to match the legend order.
+  if (reverse) {
+    data <- data[order(data$xmin, data$group), ]
+  } else {
+    data <- data[order(data$xmin, -data$group), ]
+  }
+
 
   # Check for overlap
   intervals <- as.numeric(t(unique(data[c("xmin", "xmax")])))

R/position-stack.r

@@ -21,6 +21,8 @@
 #'   (like points or lines), not a dimension (like bars or areas). Set to
 #'   \code{0} to align with the bottom, \code{0.5} for the middle,
 #'   and \code{1} (the default) for the top.
+#' @param reverse If \code{TRUE}, will reverse the default stacking order.
+#'   This is useful if you're rotating both the plot and legend.
 #' @seealso See \code{\link{geom_bar}}, and \code{\link{geom_area}} for
 #'   more examples.
 #' @export
@@ -96,21 +98,21 @@
 #'   "b", -1, "y"
 #' )
 #' ggplot(data = df, aes(x, y, group = grp)) +
-#'   geom_col(aes(fill = grp)) +
+#'   geom_col(aes(fill = grp), position = position_stack(reverse = TRUE)) +
 #'   geom_hline(yintercept = 0)
 #'
 #' ggplot(data = df, aes(x, y, group = grp)) +
 #'   geom_col(aes(fill = grp)) +
 #'   geom_hline(yintercept = 0) +
 #'   geom_text(aes(label = grp), position = position_stack(vjust = 0.5))
-position_stack <- function(vjust = 1) {
-  ggproto(NULL, PositionStack, vjust = vjust)
+position_stack <- function(vjust = 1, reverse = FALSE) {
+  ggproto(NULL, PositionStack, vjust = vjust, reverse = reverse)
 }
 
 #' @export
 #' @rdname position_stack
-position_fill <- function(vjust = 1) {
-  ggproto(NULL, PositionFill, vjust = vjust)
+position_fill <- function(vjust = 1, reverse = FALSE) {
+  ggproto(NULL, PositionFill, vjust = vjust, reverse = reverse)
 }
 
 #' @rdname ggplot2-ggproto
@@ -121,12 +123,14 @@ PositionStack <- ggproto("PositionStack", Position,
   type = NULL,
   vjust = 1,
   fill = FALSE,
+  reverse = FALSE,
 
   setup_params = function(self, data) {
     list(
       var = self$var %||% stack_var(data),
       fill = self$fill,
-      vjust = self$vjust
+      vjust = self$vjust,
+      reverse = self$reverse
     )
   },
 
@@ -157,20 +161,17 @@ PositionStack <- ggproto("PositionStack", Position,
     pos <- data[!negative, , drop = FALSE]
 
     if (any(negative)) {
-      # Negate group so sorting order is consistent across the x-axis.
-      # Undo negation afterwards so it doesn't mess up the rest
-      neg$group <- -neg$group
       neg <- collide(neg, NULL, "position_stack", pos_stack,
         vjust = params$vjust,
-        fill = params$fill
+        fill = params$fill,
+        reverse = params$reverse
       )
-      neg$group <- -neg$group
     }
-
     if (any(!negative)) {
       pos <- collide(pos, NULL, "position_stack", pos_stack,
         vjust = params$vjust,
-        fill = params$fill
+        fill = params$fill,
+        reverse = params$reverse
       )
     }
 

tests/testthat/test-position-stack.R

@@ -28,6 +28,17 @@ test_that("negative and positive values are handled separately", {
   expect_equal(dat$ymax[dat$x == 2], c(0, 2))
 })
 
+test_that("can request reverse stacking", {
+  df <- data.frame(
+    y = c(-2, 2, -1, 1),
+    g = c("a", "a", "b", "b")
+  )
+  p <- ggplot(df, aes(1, y, fill = g)) +
+    geom_col(position = position_stack(reverse = TRUE))
+  dat <- layer_data(p)
+  expect_equal(dat$ymin, c(-2, -3, 0, 2))
+})
+
 test_that("data with no extent is stacked correctly", {
   df = data.frame(
     x = c(1, 1),
@@ -38,6 +49,6 @@ test_that("data with no extent is stacked correctly", {
   p0 <- base + geom_text(aes(label = y), position = position_stack(vjust = 0))
   p1 <- base + geom_text(aes(label = y), position = position_stack(vjust = 1))
 
-  expect_equal(layer_data(p0)$y, c(-40, -115))
-  expect_equal(layer_data(p1)$y, c(0, -40))
+  expect_equal(layer_data(p0)$y, c(-75, -115))
+  expect_equal(layer_data(p1)$y, c(0, -75))
 })