Give midpoint argument to gradient scales

NEWS.md

@@ -1,6 +1,9 @@
 
 # ggplot2 (development version)
 
+* `scale_{fill/colour}_gradientn()` and `scale_{fill/colour}_stepsn()` gain
+ a `midpoint` argument to make it easier to build divergent scales 
+ (@teunbrand, #3738).
 * When facets coerce the faceting variables to factors, the 'ordered' class
  is dropped (@teunbrand, #5666).
 * `update_geom_defaults()` and `update_stat_defaults()` have a reset mechanism

R/scale-gradient.R

@@ -103,8 +103,9 @@ scale_fill_gradient <- function(name = waiver(), ..., low = "#132B43",
 
 #' @inheritParams scales::pal_div_gradient
 #' @inheritParams continuous_scale
-#' @param midpoint The midpoint (in data value) of the diverging scale.
-#' Defaults to 0.
+#' @param midpoint The midpoint (in data value) for diverging scales.
+#' Can be set to `NULL` to skip centering the palette. The `midpoint` argument
+#' is incompatible with setting a `rescaler` argument.
 #' @rdname scale_gradient
 #' @export
 scale_colour_gradient2 <- function(name = waiver(), ..., low = muted("red"),
@@ -137,6 +138,9 @@ scale_fill_gradient2 <- function(name = waiver(), ..., low = muted("red"),
 
 mid_rescaler <- function(mid, transform = "identity",
  arg = caller_arg(mid), call = caller_env()) {
+ if (is.null(mid)) {
+ return(rescale)
+ }
  transform <- as.trans(transform)
  trans_mid <- transform$transform(mid)
  check_transformation(
@@ -155,26 +159,30 @@ mid_rescaler <- function(mid, transform = "identity",
 scale_colour_gradientn <- function(name = waiver(), ..., colours, values = NULL,
  space = "Lab", na.value = "grey50",
  guide = "colourbar", aesthetics = "colour",
- colors) {
+ midpoint = NULL, transform = "identity",
+ rescaler = NULL, colors) {
  colours <- if (missing(colours)) colors else colours
 
  continuous_scale(
  aesthetics, name = name,
  palette = pal_gradient_n(colours, values, space),
- na.value = na.value, guide = guide, ...
+ rescaler = rescaler %||% mid_rescaler(midpoint, transform),
+ na.value = na.value, guide = guide, transform = transform, ...
  )
 }
 #' @rdname scale_gradient
 #' @export
 scale_fill_gradientn <- function(name = waiver(), ..., colours, values = NULL,
  space = "Lab", na.value = "grey50",
  guide = "colourbar", aesthetics = "fill",
- colors) {
+ midpoint = NULL, transform = "identity",
+ rescaler = NULL, colors) {
  colours <- if (missing(colours)) colors else colours
 
  continuous_scale(
  aesthetics, name = name,
  palette = pal_gradient_n(colours, values, space),
- na.value = na.value, guide = guide, ...
+ rescaler = rescaler %||% mid_rescaler(midpoint, transform),
+ na.value = na.value, guide = guide, transform = transform, ...
  )
 }

R/scale-steps.R

@@ -77,12 +77,14 @@ scale_colour_steps2 <- function(name = waiver(), ..., low = muted("red"),
 scale_colour_stepsn <- function(name = waiver(), ..., colours, values = NULL,
  space = "Lab", na.value = "grey50",
  guide = "coloursteps", aesthetics = "colour",
- colors) {
+ midpoint = NULL, transform = "identity",
+ rescaler = NULL, colors) {
  colours <- if (missing(colours)) colors else colours
  binned_scale(
  aesthetics, name = name,
  palette = pal_gradient_n(colours, values, space),
- na.value = na.value, guide = guide,
+ na.value = na.value, guide = guide, transform = transform,
+ rescaler = rescaler %||% mid_rescaler(midpoint, transform),
  ...
  )
 }
@@ -119,11 +121,14 @@ scale_fill_steps2 <- function(name = waiver(), ..., low = muted("red"),
 scale_fill_stepsn <- function(name = waiver(), ..., colours, values = NULL,
  space = "Lab", na.value = "grey50",
  guide = "coloursteps", aesthetics = "fill",
- colors) {
+ transform = "identity", midpoint = NULL,
+ rescaler = NULL, colors) {
  colours <- if (missing(colours)) colors else colours
  binned_scale(
  aesthetics, name = name,
  palette = pal_gradient_n(colours, values, space),
- na.value = na.value, guide = guide, ...
+ na.value = na.value, guide = guide, transform = transform,
+ rescaler = rescaler %||% mid_rescaler(midpoint, transform),
+ ...
  )
 }