Fix for raster resolution (#5412)

* coord transformation after determining positions

* add tolerance to `resolution()`

* Add tests

* add news bullet

Author: teunbrand

NEWS.md

@@ -1,7 +1,11 @@
 # ggplot2 (development version)
 
+* `resolution()` has a small tolerance, preventing spuriously small resolutions 
+  due to rounding errors (@teunbrand, #2516).
+
 * `stage()` now works correctly, even with aesthetics that do not have scales 
   (#5408)
+
 * `labeller()` now handles unspecified entries from lookup tables
   (@92amartins, #4599).
 

R/geom-raster.R

@@ -92,12 +92,13 @@ GeomRaster <- ggproto("GeomRaster", Geom,
         "{.fn {snake_class(self)}} only works with {.fn coord_cartesian}"
       ))
     }
-    data <- coord$transform(data, panel_params)
 
     # Convert vector of data to raster
     x_pos <- as.integer((data$x - min(data$x)) / resolution(data$x, FALSE))
     y_pos <- as.integer((data$y - min(data$y)) / resolution(data$y, FALSE))
 
+    data <- coord$transform(data, panel_params)
+
     nrow <- max(y_pos) + 1
     ncol <- max(x_pos) + 1
 

R/utilities-resolution.R

@@ -27,6 +27,7 @@ resolution <- function(x, zero = TRUE) {
   if (zero) {
     x <- unique0(c(0, x))
   }
-
-  min(diff(sort(x)))
+  d <- diff(sort(x))
+  tolerance <- sqrt(.Machine$double.eps)
+  min(d[d > tolerance])
 }

tests/testthat/_snaps/geom-raster/discrete-positions.svg

@@ -68,4 +68,11 @@ test_that("geom_raster draws correctly", {
   expect_doppelganger("irregular categorical",
     ggplot(df, aes(x, y, fill = factor(col))) + geom_raster()
   )
+
+  # Categorical axes -----------------------------------------------------------
+
+  df <- expand.grid(x = c("A", "B"), y = c("C", "D"))
+  expect_doppelganger("discrete positions",
+    ggplot(df, aes(x, y, fill = interaction(x, y))) + geom_raster()
+  )
 })

tests/testthat/test-geom-raster.R

@@ -173,4 +173,7 @@ test_that("resolution() gives correct answers", {
   expect_equal(resolution(mapped_discrete(c(4, 6))), 1L)
   expect_equal(resolution(c(0, 0)), 1L)
   expect_equal(resolution(c(0.5,  1.5), zero = TRUE), 0.5)
+
+  # resolution has a tolerance
+  expect_equal(resolution(c(1, 1 + 1000 * .Machine$double.eps, 2)), 1)
 })