Continuous limits for discrete scales (#6273)

* simplify expansion

* add `continuous.limits` as scale argument

* allow `scale$continuous_limits` to overrule expansion limits

* document

* confine and improve

* add news bullet

* add tests

* fix wrong description

* Better parity with continuous limits

Author: teunbrand

NEWS.md

@@ -1,5 +1,7 @@
 # ggplot2 (development version)
 
+* `scale_{x/y}_discrete(continuous.limits)` is a new argument to control the
+  display range of discrete scales (@teunbrand, #4174, #6259).
 * `geom_ribbon()` now appropriately warns about, and removes, missing values 
   (@teunbrand, #6243).
 * `guide_*()` can now accept two inside legend theme elements:

R/scale-discrete-.R

@@ -16,6 +16,12 @@
 #'   argument (the number of levels in the scale) returns the numerical values
 #'   that they should take.
 #' @param sec.axis [dup_axis()] is used to specify a secondary axis.
+#' @param continuous.limits One of:
+#'   * `NULL` to use the default scale range
+#'   * A numeric vector of length two providing a display range for the scale.
+#'   Use `NA` to refer to the existing minimum or maximum.
+#'   * A function that accepts the limits and returns a numeric vector of
+#'   length two.
 #' @rdname scale_discrete
 #' @family position scales
 #' @seealso
@@ -69,7 +75,8 @@
 #' }
 scale_x_discrete <- function(name = waiver(), ..., palette = seq_len,
                              expand = waiver(), guide = waiver(),
-                             position = "bottom", sec.axis = waiver()) {
+                             position = "bottom", sec.axis = waiver(),
+                             continuous.limits = NULL) {
   sc <- discrete_scale(
     aesthetics = ggplot_global$x_aes, name = name,
     palette = palette, ...,
@@ -78,13 +85,15 @@ scale_x_discrete <- function(name = waiver(), ..., palette = seq_len,
   )
 
   sc$range_c <- ContinuousRange$new()
+  sc$continuous_limits <- continuous.limits
   set_sec_axis(sec.axis, sc)
 }
 #' @rdname scale_discrete
 #' @export
 scale_y_discrete <- function(name = waiver(), ..., palette = seq_len,
                              expand = waiver(), guide = waiver(),
-                             position = "left", sec.axis = waiver()) {
+                             position = "left", sec.axis = waiver(),
+                             continuous.limits = NULL) {
   sc <- discrete_scale(
     aesthetics = ggplot_global$y_aes, name = name,
     palette = palette, ...,
@@ -93,6 +102,7 @@ scale_y_discrete <- function(name = waiver(), ..., palette = seq_len,
   )
 
   sc$range_c <- ContinuousRange$new()
+  sc$continuous_limits <- continuous.limits
   set_sec_axis(sec.axis, sc)
 }
 
@@ -106,6 +116,8 @@ scale_y_discrete <- function(name = waiver(), ..., palette = seq_len,
 #' @usage NULL
 #' @export
 ScaleDiscretePosition <- ggproto("ScaleDiscretePosition", ScaleDiscrete,
+  continuous_limits = NULL,
+
   train = function(self, x) {
     if (is.discrete(x)) {
       self$range$train(x, drop = self$drop, na.rm = !self$na.translate)

R/scale-expansion.R

@@ -81,9 +81,7 @@ expand_range4 <- function(limits, expand) {
 
   # Calculate separate range expansion for the lower and
   # upper range limits, and then combine them into one vector
-  lower <- expand_range(limits, expand[1], expand[2])[1]
-  upper <- expand_range(limits, expand[3], expand[4])[2]
-  c(lower, upper)
+  expand_range(limits, expand[c(1, 3)], expand[c(2, 4)])
 }
 
 #' Calculate the default expansion for a scale
@@ -153,7 +151,8 @@ expand_limits_scale <- function(scale, expand = expansion(0, 0), limits = waiver
       scale$map(limits),
       expand,
       coord_limits,
-      range_continuous = scale$range_c$range
+      range_continuous = scale$range_c$range,
+      continuous_limits = scale$continuous_limits
     )
   } else {
     # using the inverse transform to resolve the NA value is needed for date/datetime/time
@@ -170,7 +169,20 @@ expand_limits_continuous <- function(limits, expand = expansion(0, 0), coord_lim
 }
 
 expand_limits_discrete <- function(limits, expand = expansion(0, 0), coord_limits = c(NA, NA),
-                                   range_continuous = NULL) {
+                                   range_continuous = NULL, continuous_limits = NULL) {
+  if (is.function(continuous_limits)) {
+    continuous_limits <- continuous_limits(limits)
+  }
+  if (!is.null(continuous_limits)) {
+    if (!anyNA(continuous_limits)) {
+      continuous_limits <- range(continuous_limits)
+    }
+    check_numeric(continuous_limits, arg = "continuous.limits")
+    check_length(continuous_limits, 2L, arg = "continuous.limits")
+    missing <- is.na(continuous_limits)
+    limits  <- ifelse(missing, range(limits), continuous_limits)
+  }
+
   limit_info <- expand_limits_discrete_trans(
     limits,
     expand,

man/scale_discrete.Rd

@@ -96,6 +96,27 @@ test_that("expand_limits_discrete() can override limits with a both discrete and
     expand_limits_discrete(c("one", "two"), coord_limits = c(0, NA), range_continuous = c(1, 2)),
     c(0, 2)
   )
+  expect_identical(
+    expand_limits_discrete(1:2, range_continuous = c(1, 2), continuous_limits = c(0, 3)),
+    c(0, 3)
+  )
+  expect_identical(
+    expand_limits_discrete(1:2, range_continuous = c(1, 2), continuous_limits = c(NA, 4)),
+    c(1, 4)
+  )
+  expect_identical(
+    expand_limits_discrete(1:2, range_continuous = c(1, 2), continuous_limits = c(0, NA)),
+    c(0, 2)
+  )
+  expect_identical(
+    expand_limits_discrete(1:2, range_continuous = c(1, 2), continuous_limits = c(NA_real_, NA_real_)),
+    c(1, 2)
+  )
+  expect_identical(
+    expand_limits_discrete(1:2, range_continuous = 1:2,
+                           continuous_limits = function(x) x + c(-1, 1)),
+    c(0, 3)
+  )
 })
 
 test_that("expand_limits_continuous_trans() works with inverted transformations", {