n.breaks propagate to sec.axis

NEWS.md

@@ -1,5 +1,6 @@
 # ggplot2 (development version)
 
+* Secondary axes respect `n.breaks` setting in continuous scales (@teunbrand, #4483).
 * `stat_bin()` now accepts functions for argument `breaks` (@aijordan, #4561)
 * (internal) The plot's layout now has a coord parameter that is used to 
   prevent setting up identical panel parameters (#5427)

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)))
+})