Allow geom_hexbin to use ..density..

Fixes #1608. Closes #1688. By @mikebirdgeneau

NEWS.md

@@ -1,5 +1,8 @@
 # ggplot2 2.1.0.9000 
 
+* Restore functionality for use of `..density..` in 
+  `geom_hexbin()` (@mikebirdgeneau, #1688)
+
 * `stat_smooth()` once again informs you about the method it has chosen.
   It also correctly calculates the size of the largest group within facets.
 

R/stat-binhex.r

@@ -46,7 +46,10 @@ StatBinhex <- ggproto("StatBinhex", Stat,
 
     binwidth <- binwidth %||% hex_binwidth(bins, scales)
     wt <- data$weight %||% rep(1L, nrow(data))
-    hexBinSummarise(data$x, data$y, wt, binwidth, sum)
+    out <- hexBinSummarise(data$x, data$y, wt, binwidth, sum)
+    out$density <- as.vector(out$value / sum(out$value, na.rm = TRUE))
+
+    out
   }
 )
 

tests/testthat/test-geom-hex.R

@@ -0,0 +1,12 @@
+context("geom_hex")
+
+test_that("density and value summaries available", {
+  df <- data.frame(x = c(1, 1, 1, 2), y = c(1, 1, 1, 2))
+  base <- ggplot(df, aes(x, y)) +
+    geom_hex()
+
+  out <- layer_data(base)
+  expect_equal(nrow(out), 2)
+  expect_equal(out$density, c(0.75, 0.25), tolerance = 1e-7)
+  expect_equal(out$value, c(3, 1), tolerance = 1e-7)
+})