Add bin_prop computed variable to stat_bin

Brings feature parity with stat_count by adding `after_stat(bin_prop)`
functionality to stat_bin. The bin_prop variable shows the proportion
of each group within each bin, enabling proportion-based visualizations
for binned continuous data.

Key features:
- bin_prop = count_in_group / total_count_in_bin
- Works with multiple groups and respects weights
- Backwards compatible (bin_prop = 1 for single groups)
- Properly handles empty bins

Usage:
ggplot(data, aes(x = continuous_var, y = after_stat(bin_prop), fill = group)) +
  stat_bin(geom = "col", position = "dodge")

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: kieran-mace

R/stat-bin.R

@@ -63,6 +63,54 @@ StatBin <- ggproto(
     flip_data(bins, flipped_aes)
   },
 
+  compute_panel = function(self, data, scales, binwidth = NULL, bins = NULL,
+                           center = NULL, boundary = NULL,
+                           closed = c("right", "left"), pad = FALSE,
+                           breaks = NULL, flipped_aes = FALSE, drop = "none") {
+    # First call parent's compute_panel to get binned data for all groups
+    data <- ggproto_parent(Stat, self)$compute_panel(
+      data, scales, binwidth = binwidth, bins = bins,
+      center = center, boundary = boundary, closed = closed,
+      pad = pad, breaks = breaks, flipped_aes = flipped_aes, drop = drop
+    )
+    
+    # Only calculate bin_prop if we have the necessary columns and multiple groups
+    if (!is.null(data) && nrow(data) > 0 && 
+        all(c("count", "xmin", "xmax") %in% names(data))) {
+      
+      # Calculate bin_prop: proportion of each group within each bin
+      # Create a unique bin identifier using rounded values to handle floating point precision
+      data$bin_id <- paste(round(data$xmin, 10), round(data$xmax, 10), sep = "_")
+      
+      # Calculate total count per bin across all groups
+      bin_totals <- stats::aggregate(data$count, by = list(bin_id = data$bin_id), FUN = sum)
+      names(bin_totals)[2] <- "bin_total"
+      
+      # Merge back to get bin totals for each row
+      data <- merge(data, bin_totals, by = "bin_id", sort = FALSE)
+      
+      # Calculate bin_prop: count within group / total count in bin
+      # When bin_total = 0 (empty bin), set bin_prop based on whether there are multiple groups
+      n_groups <- length(unique(data$group))
+      if (n_groups == 1) {
+        # With only one group, bin_prop is always 1 (100% of the bin belongs to this group)
+        data$bin_prop <- 1
+      } else {
+        # With multiple groups, bin_prop = count / total_count_in_bin, or 0 for empty bins
+        data$bin_prop <- ifelse(data$bin_total > 0, data$count / data$bin_total, 0)
+      }
+      
+      # Remove the temporary columns
+      data$bin_id <- NULL
+      data$bin_total <- NULL
+    } else {
+      # If we don't have the necessary data, just add a default bin_prop column
+      data$bin_prop <- if (nrow(data) > 0) rep(1, nrow(data)) else numeric(0)
+    }
+    
+    data
+  },
+
   default_aes = aes(x = after_stat(count), y = after_stat(count), weight = 1),
 
   required_aes = "x|y",
@@ -108,7 +156,8 @@ StatBin <- ggproto(
 #'   density  = "density of points in bin, scaled to integrate to 1.",
 #'   ncount   = "count, scaled to a maximum of 1.",
 #'   ndensity = "density, scaled to a maximum of 1.",
-#'   width    = "widths of bins."
+#'   width    = "widths of bins.",
+#'   bin_prop = "proportion of points in bin that belong to each group."
 #' )
 #'
 #' @section Dropped variables:

tests/testthat/test-stat-bin.R

@@ -257,3 +257,77 @@ test_that("stat_count preserves x order for continuous and discrete", {
   expect_identical(b$layout$panel_params[[1]]$x$get_labels(), c("4","1","2","3","6","8"))
   expect_identical(b$data[[1]]$y, c(10,7,10,3,1,1))
 })
+
+# Test bin_prop functionality ---------------------------------------------
+
+test_that("stat_bin calculates bin_prop correctly", {
+  # Create test data with two distinct groups
+  test_data <- data_frame(
+    x = c(rep(c(1, 2, 3, 4, 5), each = 10), rep(c(3, 4, 5, 6, 7), each = 10)),
+    group = rep(c("A", "B"), each = 50)
+  )
+  
+  # Test with 5 bins to get predictable overlap
+  p <- ggplot(test_data, aes(x, fill = group)) + geom_histogram(bins = 5)
+  data <- get_layer_data(p)
+  
+  # bin_prop should be available
+  expect_true("bin_prop" %in% names(data))
+  
+  # All bin_prop values should be between 0 and 1
+  expect_true(all(data$bin_prop >= 0 & data$bin_prop <= 1))
+  
+  # For bins that contain both groups, bin_prop should sum to 1 across groups
+  bins_with_both_groups <- aggregate(data$count > 0, by = list(paste(data$xmin, data$xmax)), sum)
+  overlapping_bins <- bins_with_both_groups[bins_with_both_groups$x == 2, ]$Group.1
+  
+  for (bin in overlapping_bins) {
+    bin_data <- data[paste(data$xmin, data$xmax) == bin, ]
+    total_prop <- sum(bin_data$bin_prop)
+    expect_equal(total_prop, 1, tolerance = 1e-6)
+  }
+  
+  # Test after_stat(bin_prop) usage
+  p2 <- ggplot(test_data, aes(x, y = after_stat(bin_prop), fill = group)) + 
+    stat_bin(geom = "col", bins = 5, position = "dodge")
+  data2 <- get_layer_data(p2)
+  
+  # y should contain the bin_prop values
+  expect_equal(data2$y, data2$bin_prop)
+})
+
+test_that("stat_bin bin_prop works with single group", {
+  # Test that bin_prop = 1 for all bins when there's only one group
+  test_data <- data_frame(x = c(1, 2, 3, 4, 5, 6, 7, 8, 9, 10))
+  
+  p <- ggplot(test_data, aes(x)) + geom_histogram(bins = 5)
+  data <- get_layer_data(p)
+  
+  # All bin_prop values should be 1 (even for empty bins)
+  expect_true(all(data$bin_prop == 1))
+})
+
+test_that("stat_bin bin_prop works with weights", {
+  # Test that bin_prop calculation respects weights
+  test_data <- data_frame(
+    x = c(1, 1, 2, 2),
+    group = c("A", "B", "A", "B"),
+    w = c(1, 3, 2, 2)  # Different weights for each observation
+  )
+  
+  p <- ggplot(test_data, aes(x, fill = group, weight = w)) + geom_histogram(bins = 2)
+  data <- get_layer_data(p)
+  
+  # bin_prop should be available
+  expect_true("bin_prop" %in% names(data))
+  
+  # Check that proportions are calculated correctly with weights
+  # Bin 1: A=1, B=3, total=4, so A should have bin_prop=0.25, B should have bin_prop=0.75
+  # Bin 2: A=2, B=2, total=4, so A should have bin_prop=0.5, B should have bin_prop=0.5
+  bin1_data <- data[data$x == min(data$x), ]
+  bin2_data <- data[data$x == max(data$x), ]
+  
+  # For each bin, proportions should sum to 1
+  expect_equal(sum(bin1_data$bin_prop), 1, tolerance = 1e-6)
+  expect_equal(sum(bin2_data$bin_prop), 1, tolerance = 1e-6)
+})