Sort within x prior to stacking

NEWS.md

@@ -1,5 +1,13 @@
 # ggplot2 2.1.0.9000 
 
+* `position_stack()` and `position_fill()` now sorts the stacking order so it 
+  matches the order of the grouping. Use level reordering to alter the stacking 
+  order. The default legend and stacking order is now also in line. The default 
+  look of plots might change because of this (#1552, #1593).
+
+* `position_stack()` now accepts negative values which will create stacks 
+  extending below the x-axis (#1691)
+
 * Restore functionality for use of `..density..` in 
   `geom_hexbin()` (@mikebirdgeneau, #1688)
 

R/position-collide.r

@@ -26,9 +26,9 @@ collide <- function(data, width = NULL, name, strategy, check.width = TRUE) {
     width <- widths[1]
   }
 
-  # Reorder by x position, relying on stable sort to preserve existing
-  # ordering, which may be by group or order.
-  data <- data[order(data$xmin), ]
+  # Reorder by x position, then on group. Group is reversed so stacking order
+  # follows the default legend order
+  data <- data[order(data$xmin, -data$group), ]
 
   # Check for overlap
   intervals <- as.numeric(t(unique(data[c("xmin", "xmax")])))

R/position-stack.r

@@ -3,6 +3,19 @@
 #' \code{position_fill} additionally standardises each stack to have unit
 #' height.
 #'
+#' @details \code{position_fill} and \code{position_stack} automatically stacks
+#' values so their order follows the decreasing sort order of the fill
+#' aesthetic. This makes sure that the stack order is aligned with the order in
+#' the legend, as long as the scale order has not been changed using the
+#' \code{breaks} argument. This also means that in order to change stacking
+#' order while preserving parity with the legend order it is necessary to
+#' reorder the factor levels of the fill aesthetic (see examples)
+#'
+#' Stacking of positive and negative values are performed separately so that
+#' positive values stack upwards from the x-axis and negative values stack
+#' downward. Do note that parity with legend order cannot be ensured when
+#' positive and negative values are mixed.
+#'
 #' @family position adjustments
 #' @seealso See \code{\link{geom_bar}}, and \code{\link{geom_area}} for
 #'   more examples.
@@ -41,6 +54,20 @@
 #'
 #' # But realise that this makes it *much* harder to compare individual
 #' # trends
+#'
+#' # Stacking order can be changed using ordered factors
+#' data.set$Type <- factor(data.set$Type, levels = c('c', 'b', 'd', 'a'))
+#' ggplot(data.set, aes(Time, Value)) + geom_area(aes(fill = Type))
+#'
+#' # while changing the scale order won't affect the stacking
+#' ggplot(data.set, aes(Time, Value)) + geom_area(aes(fill = Type)) +
+#'   scale_fill_discrete(breaks = c('a', 'b', 'c', 'd'))
+#'
+#' # Negative values can be stacked as well
+#' neg <- data.set$Type %in% c('a', 'd')
+#' data.set$Value[neg] <- data.set$Value[neg] * -1
+#' ggplot(data.set, aes(Time, Value)) + geom_area(aes(fill = Type))
+#'
 position_stack <- function() {
   PositionStack
 }
@@ -61,14 +88,31 @@ PositionStack <- ggproto("PositionStack", Position,
         "Maybe you want position = 'identity'?")
       return(data)
     }
-
-    if (!is.null(data$ymin) && !all(data$ymin == 0))
-      warning("Stacking not well defined when ymin != 0", call. = FALSE)
+    if (!is.null(data$ymax) && !is.null(data$ymin)) {
+      switch_index <- data$ymax < data$ymin
+      data$ymin[switch_index] <- data$ymax[switch_index]
+      data$ymax[switch_index] <- 0
+    }
+    if (!is.null(data$ymin) && !all((data$ymin == 0 & data$ymax >= 0) | data$ymax == 0 & data$ymin <= 0))
+      warning("Stacking not well defined when ymin and ymax is on opposite sides of 0", call. = FALSE)
 
     data
   },
 
   compute_panel = function(data, params, scales) {
-    collide(data, NULL, "position_stack", pos_stack)
+    negative <- if (!is.null(data$ymin)) data$ymin < 0 else rep(FALSE, nrow(data))
+    neg <- data[which(negative), ]
+    pos <- data[which(!negative), ]
+    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)
+      neg$group <- -neg$group
+    }
+    if (any(!negative)) {
+      pos <- collide(pos, NULL, "position_stack", pos_stack)
+    }
+    rbind(pos, neg)
   }
 )

tests/testthat/test-position-stack.R

@@ -0,0 +1,38 @@
+context("position-stack")
+
+test_that("ymin and ymax is sorted", {
+  df <- data.frame(
+    x = rep(1:2, each = 5),
+    group = rep(1:4, length.out = 10),
+    ymin = 0,
+    ymax = sample.int(10) * sample(c(-1, 1), 10, TRUE)
+  )
+  sorted <- PositionStack$setup_data(df)
+  expect_true(all(sorted$ymax[df$ymax < 0] == 0))
+  expect_true(all(sorted$ymin[df$ymax < 0] == df$ymax[df$ymax < 0]))
+})
+
+test_that("data is sorted prior to stacking", {
+  df <- data.frame(
+    x = rep(c(1:10), 3),
+    var = rep(c("a", "b", "c"), 10),
+    y = round(runif(30, 1, 5))
+  )
+  p <- ggplot(df, aes(x = x, y = y, fill = var)) +
+    geom_area(position = "stack")
+  dat <- layer_data(p)
+  expect_true(all(dat$group == 3:1))
+})
+
+test_that("negative and positive values are handled separately", {
+  df <- data.frame(
+    x = c(1,1,1,2,2),
+    g = c(1,2,3,1,2),
+    y = c(1,-1,1,2,-3)
+  )
+  p <- ggplot(df, aes(x, y, fill= factor(g))) +
+    geom_bar(stat = "identity")
+  dat <- layer_data(p)
+  expect_equal(dat$ymin, c(0,1,0,-1,-3))
+  expect_equal(dat$ymax, c(1,2,2,0,0))
+})