Discrete minor ticks

NEWS.md

@@ -1,6 +1,9 @@
 
 # ggplot2 (development version)
 
+* Discrete scales now support `minor_breaks`. This may only make sense in
+ discrete position scales, where it affects the placement of minor ticks
+ and minor gridlines (#5434).
 * Fixed performance loss when the `.data` pronoun is used in `aes()` (#5730).
 * Fixed bug where discrete scales could not map aesthetics only consisting of
  `NA`s (#5623)

R/guide-axis.R

@@ -133,6 +133,11 @@ GuideAxis <- ggproto(
 
  if (nrow(major) > 0) {
  major$.type <- "major"
+ if (!vec_is(minor$.value, major$.value)) {
+ # If we have mixed types of values, which may happen in discrete scales,
+ # discard minor values in favour of the major values.
+ minor$.value <- NULL
+ }
  vec_rbind(major, minor)
  } else {
  minor

R/scale-.R

@@ -23,13 +23,13 @@
 #' Also accepts rlang [lambda][rlang::as_function()] function notation.
 #' @param minor_breaks One of:
 #' - `NULL` for no minor breaks
-#' - `waiver()` for the default breaks (one minor break between
-#' each major break)
+#' - `waiver()` for the default breaks (none for discrete, one minor break
+#' between each major break for continuous)
 #' - A numeric vector of positions
 #' - A function that given the limits returns a vector of minor breaks. Also
 #' accepts rlang [lambda][rlang::as_function()] function notation. When
 #' the function has two arguments, it will be given the limits and major
-#' breaks.
+#' break positions.
 #' @param n.breaks An integer guiding the number of major breaks. The algorithm
 #' may choose a slightly different number to ensure nice break labels. Will
 #' only have an effect if `breaks = waiver()`. Use `NULL` to use the default
@@ -199,7 +199,8 @@ continuous_scale <- function(aesthetics, scale_name = deprecated(), palette, nam
 #' The `r link_book("new scales section", "extensions#sec-new-scales")`
 #' @keywords internal
 discrete_scale <- function(aesthetics, scale_name = deprecated(), palette, name = waiver(),
- breaks = waiver(), labels = waiver(), limits = NULL, expand = waiver(),
+ breaks = waiver(), minor_breaks = waiver(),
+ labels = waiver(), limits = NULL, expand = waiver(),
  na.translate = TRUE, na.value = NA, drop = TRUE,
  guide = "legend", position = "left",
  call = caller_call(),
@@ -217,6 +218,7 @@ discrete_scale <- function(aesthetics, scale_name = deprecated(), palette, name
  limits <- allow_lambda(limits)
  breaks <- allow_lambda(breaks)
  labels <- allow_lambda(labels)
+ minor_breaks <- allow_lambda(minor_breaks)
 
  if (!is.function(limits) && (length(limits) > 0) && !is.discrete(limits)) {
  cli::cli_warn(c(
@@ -246,6 +248,7 @@ discrete_scale <- function(aesthetics, scale_name = deprecated(), palette, name
 
  name = name,
  breaks = breaks,
+ minor_breaks = minor_breaks,
  labels = labels,
  drop = drop,
  guide = guide,
@@ -1020,7 +1023,34 @@ ScaleDiscrete <- ggproto("ScaleDiscrete", Scale,
  structure(in_domain, pos = match(in_domain, breaks))
  },
 
- get_breaks_minor = function(...) NULL,
+ get_breaks_minor = function(self, n = 2, b = self$break_positions(),
+ limits = self$get_limits()) {
+ breaks <- self$minor_breaks
+ # The default is to draw no minor ticks
+ if (is.null(breaks %|W|% NULL)) {
+ return(NULL)
+ }
+ if (is.function(breaks)) {
+ # Ensure function gets supplied numeric limits and breaks
+ if (!is.numeric(b)) {
+ b <- self$map(b)
+ }
+ if (!is.numeric(limits)) {
+ limits <- self$map(limits)
+ limits <- self$dimension(self$expand, limits)
+ }
+
+ # Allow for two types of minor breaks specifications
+ break_fun <- fetch_ggproto(self, "minor_breaks")
+ arg_names <- fn_fmls_names(break_fun)
+ if (length(arg_names) == 1L) {
+ breaks <- break_fun(limits)
+ } else {
+ breaks <- break_fun(limits, b)
+ }
+ }
+ breaks
+ },
 
  get_labels = function(self, breaks = self$get_breaks()) {
  if (self$is_empty()) {

R/scale-view.R

@@ -15,17 +15,16 @@
 view_scale_primary <- function(scale, limits = scale$get_limits(),
  continuous_range = scale$dimension(limits = limits)) {
 
+ # continuous_range can be specified in arbitrary order, but
+ # scales expect the one in ascending order.
+ continuous_scale_sorted <- sort(continuous_range)
  if(!scale$is_discrete()) {
- # continuous_range can be specified in arbitrary order, but
- # continuous scales expect the one in ascending order.
- continuous_scale_sorted <- sort(continuous_range)
  breaks <- scale$get_breaks(continuous_scale_sorted)
- minor_breaks <- scale$get_breaks_minor(b = breaks, limits = continuous_scale_sorted)
  breaks <- censor(breaks, continuous_scale_sorted, only.finite = FALSE)
  } else {
  breaks <- scale$get_breaks(limits)
- minor_breaks <- scale$get_breaks_minor(b = breaks, limits = limits)
  }
+ minor_breaks <- scale$get_breaks_minor(b = breaks, limits = continuous_scale_sorted)
  minor_breaks <- censor(minor_breaks, continuous_range, only.finite = FALSE)
 
  ggproto(NULL, ViewScale,