Allow functional limits in continuous scales

NEWS.md

@@ -47,6 +47,11 @@ core developer team.
 
 * `x0` and `y0` are now recognized positional aesthetics so they will get scaled 
   if used in extension geoms and stats (@thomasp85, #3168)
+
+* Continuous scale limits now accept functions which accept the default
+  limits and return adjusted limits. This makes it possible to write
+  a function that e.g. ensures the limits are always a multiple of 100,
+  regardless of the data (@econandrew, #2307).
 
 ## Minor improvements and bug fixes
 

R/scale-.r

@@ -14,7 +14,6 @@
 Scale <- ggproto("Scale", NULL,
 
   call = NULL,
-
   aesthetics = aes(),
   scale_name = NULL,
   palette = function() {
@@ -107,16 +106,20 @@ Scale <- ggproto("Scale", NULL,
     stop("Not implemented", call. = FALSE)
   },
 
-  #  if scale contains a NULL, use the default scale range
+  #  if scale is a function, apply it to the default (inverted) scale range
+  #  if scale is NULL, use the default scale range
   #  if scale contains a NA, use the default range for that axis, otherwise
   #  use the user defined limit for that axis
   get_limits = function(self) {
     if (self$is_empty()) return(c(0, 1))
 
-    if (!is.null(self$limits)) {
-      ifelse(!is.na(self$limits), self$limits, self$range$range)
-    } else {
+    if (is.null(self$limits)) {
       self$range$range
+    } else if (is.function(self$limits)) {
+      # if limits is a function, it expects to work in data space
+      self$trans$transform(self$limits(self$trans$inverse(self$range$range)))
+    } else {
+      ifelse(is.na(self$limits), self$range$range, self$limits)
     }
   },
 
@@ -526,8 +529,12 @@ ScaleDiscrete <- ggproto("ScaleDiscrete", Scale,
 #'   - A character vector giving labels (must be same length as `breaks`)
 #'   - A function that takes the breaks as input and returns labels
 #'     as output
-#' @param limits A numeric vector of length two providing limits of the scale.
-#'   Use `NA` to refer to the existing minimum or maximum.
+#' @param limits One of:
+#'   - `NULL` to use the default scale range
+#'   - A numeric vector of length two providing limits of the scale.
+#'     Use `NA` to refer to the existing minimum or maximum
+#'   - A function that accepts the existing (automatic) limits and returns
+#'     new limits
 #' @param rescaler  Used by diverging and n colour gradients
 #'   (i.e. [scale_colour_gradient2()], [scale_colour_gradientn()]).
 #'   A function used to scale the input values to the range \[0, 1].
@@ -538,7 +545,7 @@ ScaleDiscrete <- ggproto("ScaleDiscrete", Scale,
 #' @param trans Either the name of a transformation object, or the
 #'   object itself. Built-in transformations include "asn", "atanh",
 #'   "boxcox", "date", "exp", "hms", "identity", "log", "log10", "log1p", "log2",
-#'   "logit", "modulus", "probability", "probit", "pseudo_log", "reciprocal", 
+#'   "logit", "modulus", "probability", "probit", "pseudo_log", "reciprocal",
 #'   "reverse", "sqrt" and "time".
 #'
 #'   A transformation object bundles together a transform, its inverse,
@@ -569,7 +576,7 @@ continuous_scale <- function(aesthetics, scale_name, palette, name = waiver(),
   }
 
   trans <- as.trans(trans)
-  if (!is.null(limits)) {
+  if (!is.null(limits) && !is.function(limits)) {
     limits <- trans$transform(limits)
   }
 

tests/testthat/test-scales-breaks-labels.r

@@ -202,7 +202,6 @@ test_that("scale_breaks with explicit NA options (deprecated)", {
   expect_error(scc$get_breaks())
 })
 
-
 test_that("breaks can be specified by names of labels", {
   labels <- setNames(LETTERS[1:4], letters[1:4])
 
@@ -241,6 +240,12 @@ test_that("minor breaks are transformed by scales", {
   expect_equal(sc$get_breaks_minor(), c(0, 1, 2))
 })
 
+test_that("continuous limits accepts functions", {
+  p <- ggplot(mpg, aes(class, hwy)) +
+    scale_y_continuous(limits = function(lims) (c(lims[1] - 10, lims[2] + 100)))
+
+  expect_equal(layer_scales(p)$y$get_limits(), c(range(mpg$hwy)[1] - 10, range(mpg$hwy)[2] + 100))
+})
 
 # Visual tests ------------------------------------------------------------
 
@@ -324,3 +329,18 @@ test_that("scale breaks can be removed", {
     ggplot(dat, aes(x = 1, y = y, colour = x)) + geom_point() + scale_colour_continuous(breaks = NULL)
   )
 })
+
+test_that("functional limits work for continuous scales", {
+  limiter <- function(by) {
+    function(limits) {
+      low <- floor(limits[1] / by) * by
+      high <- ceiling(limits[2] / by) * by
+      c(low, high)
+    }
+  }
+
+  expect_doppelganger(
+    "functional limits",
+    ggplot(mpg, aes(class)) + geom_bar(aes(fill = drv)) + scale_y_continuous(limits = limiter(50))
+  )
+})