n.breaks propagate to sec.axis (#5973)

* capture `n.breaks` parameter upon initialisation

* add test

* add news bullet

NEWS.md

@@ -1,5 +1,6 @@
 # ggplot2 (development version)
 
+* Secondary axes respect `n.breaks` setting in continuous scales (@teunbrand, #4483).
 * Layers can have names (@teunbrand, #4066).
 * (internal) improvements to `pal_qualitative()` (@teunbrand, #5013)
 * `coord_radial(clip = "on")` clips to the panel area when the graphics device

R/axis-secondary.R

@@ -188,7 +188,13 @@ AxisSecondary <- ggproto("AxisSecondary", NULL,
  if (scale$is_discrete()) {
  self$breaks <- scale$get_breaks()
  } else {
- self$breaks <- scale$get_transformation()$breaks
+ breaks <- scale$get_transformation()$breaks
+ n_breaks <- scale$n.breaks
+ if (!is.null(n_breaks) && "n" %in% fn_fmls_names(breaks)) {
+ self$breaks <- function(x) breaks(x, n = n_breaks)
+ } else {
+ self$breaks <- breaks
+ }
  }
  }
  if (is.derived(self$labels)) self$labels <- scale$labels

tests/testthat/test-sec-axis.R

@@ -400,3 +400,22 @@ test_that("discrete scales can have secondary axes", {
  expect_equal(y$.value, c(1.5, 2.5), ignore_attr = TRUE)
  expect_equal(y$.label, c("grault", "garply"))
 })
+
+test_that("n.breaks is respected by secondary axes (#4483)", {
+
+ b <- ggplot_build(
+ ggplot(data.frame(x = c(0, 10)), aes(x, x)) +
+ scale_y_continuous(
+ n.breaks = 11,
+ sec.axis = sec_axis(~.x*100)
+ )
+ )
+
+ # We get scale breaks via guide data
+ prim <- get_guide_data(b, "y")
+ sec <- get_guide_data(b, "y.sec")
+
+ expect_equal(prim$.value, sec$.value) # .value is in primary scale
+ expect_equal(prim$.label, as.character(seq(0, 10, length.out = 11)))
+ expect_equal(sec$.label, as.character(seq(0, 1000, length.out = 11)))
+})