Fix incorrect monotonicity test failure in sec.axis (#2805)

Author: Dana Paige Seidel

R/axis-secondary.R

@@ -113,10 +113,10 @@ AxisSecondary <- ggproto("AxisSecondary", NULL,
     if (self$empty()) return()
 
     # Get original range before transformation
-    along_range <- seq(range[1], range[2], length.out = self$detail)
-    old_range <- scale$trans$inverse(along_range)
+    inv_range <- scale$trans$inverse(range)
 
     # Create mapping between primary and secondary range
+    old_range <- seq(inv_range[1], inv_range[2], length.out = self$detail)
     full_range <- self$transform_range(old_range)
 
     # Test for monotonicity

tests/testthat/test-sec-axis.R

@@ -115,3 +115,19 @@ test_that("sec axis works with tidy eval", {
 
   expect_equal(breaks$major_source / 10, breaks$sec.major_source)
 })
+
+test_that("sec_axis() works for power transformations (monotonicity test doesn't fail)", {
+  p <- ggplot(foo, aes(x, y)) +
+    geom_point() +
+    scale_x_sqrt(sec.axis = dup_axis())
+  scale <- layer_scales(p)$x
+  breaks <- scale$break_info()
+  expect_equal(breaks$major, breaks$sec.major, tolerance = .001)
+
+  p <- ggplot(foo, aes(x, y)) +
+    geom_point() +
+    scale_x_sqrt(sec.axis = sec_axis(~. * 100))
+  scale <- layer_scales(p)$x
+  breaks <- scale$break_info()
+  expect_equal(breaks$major, breaks$sec.major, tolerance = .001)
+})