Allow functional limits in continuous scales

NEWS.md

@@ -24,7 +24,7 @@ core developer team.
     equivalents.
 
 * `geom_polygon()` can now draw polygons with holes using the new `subgroup` 
-  aesthetic. This functionality requires R 3.6.0 (@thomasp85, #3128)
+  aesthetic. This functionality requires R 3.6.0 (@thomasp85, #3128).
 
 * Aesthetic mappings now accept functions that return `NULL` (@yutannihilation,
   #2997).
@@ -46,12 +46,17 @@ core developer team.
   interest to extension developers (@clauswilke, #2872).
 
 * `x0` and `y0` are now recognized positional aesthetics so they will get scaled 
-  if used in extension geoms and stats (@thomasp85, #3168)
+  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
 
 * `cut_width()` now accepts `...` to pass further arguments to `base::cut.default()`
-   like `cut_number()` and `cut_interval()` already did (@cderv, #3055)
+   like `cut_number()` and `cut_interval()` already did (@cderv, #3055).
 
 * `coord_map()` now can have axes on the top and right (@karawoo, #3042).
 
@@ -83,7 +88,7 @@ core developer team.
 
 * `geom_area()` and `geom_ribbon()` now sort the data along the x-axis in the 
   `setup_data()` method rather than as part of `draw_group()` (@thomasp85, 
-  #3023)
+  #3023).
 
 * `geom_hline()`, `geom_vline()`, and `geom_abline()` now throw a warning if the 
   user supplies both an `xintercept`, `yintercept`, or `slope` value and a 
@@ -106,7 +111,7 @@ core developer team.
   #3079).
 
 * `scale_shape_identity()` now works correctly with `guide = "legend"` 
-  (@malcolmbarrett, #3029)
+  (@malcolmbarrett, #3029).
 
 * `stat_bin()` will now error when the number of bins exceeds 1e6 to avoid 
   accidentally freezing the user session (@thomasp85).
@@ -116,8 +121,7 @@ core developer team.
 * `facet_wrap()` and `facet_grid()` now automatically remove NULL from facet
   specs, and accept empty specs (@yutannihilation, #3070, #2986).
 
-* `stat_bin()` now handles data with only one unique value (@yutannihilation 
-  #3047).
+* `stat_bin()` now handles data with only one unique value (@yutannihilation, #3047).
 
 * `sec_axis()` now accepts functions as well as formulas (@yutannihilation, #3031).
 

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 is a function, apply it to the default (inverted) scale range
   #  if scale contains a 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)
   }