Making copies of trained position scales

NEWS.md

@@ -320,6 +320,8 @@
   (@teunbrand, #5093).
 * (internal) When `validate_subclass()` fails to find a class directly, it tries 
   to retrieve the class via constructor functions (@teunbrand).
+* (internal) The ViewScale class has a `make_fixed_copy()` method to permit 
+  copying trained position scales (#3441).
 
 # ggplot2 3.5.1
 

R/scale-view.R

@@ -150,5 +150,31 @@ ViewScale <- ggproto("ViewScale", NULL,
     }
 
     self$rescale(b)
+  },
+  make_fixed_copy = function(self) {
+    breaks <- self$get_breaks()
+    minor  <- self$get_breaks_minor()
+    transform <- self$scale$get_transformation()
+
+    if (self$scale$is_discrete()) {
+      limits <- self$get_limits()
+    } else {
+      limits <- self$continuous_range
+    }
+
+    if (!is.null(transform)) {
+      breaks <- transform$inverse(breaks)
+      minor  <- transform$inverse(minor)
+    }
+
+    ggproto(
+      NULL, self$scale,
+      breaks = breaks,
+      minor_breaks = minor,
+      limits = limits,
+      expand = c(0, 0, 0, 0),
+      continuous_limits = self$continuous_range,
+      train = function (...) NULL
+    )
   }
 )

tests/testthat/test-scales.R

@@ -748,6 +748,32 @@ test_that("discrete scales work with NAs in arbitrary positions", {
 
 })
 
+test_that("ViewScales can make fixed copies", {
+
+  p1 <- ggplot(mpg, aes(drv, displ)) +
+    geom_boxplot() +
+    annotate("point", x = 5, y = 10) +
+    scale_x_discrete(labels = c("four-wheel", "forward", "reverse"))
+
+  b1 <- ggplot_build(p1)$layout$panel_params[[1]]
+
+  # We build a second plot with the first plot's scales
+  p2 <- ggplot(mpg, aes(drv, cyl)) +
+    geom_violin() +
+    annotate("point", x = 15, y = 100) +
+    b1$x$make_fixed_copy() +
+    b1$y$make_fixed_copy()
+  b2 <- ggplot_build(p2)
+
+  # Breaks and labels should respect p1's limits
+  x <- get_guide_data(b2, "x")
+  expect_equal(x$x, 0.6:2.6 / diff(b1$x.range))
+  expect_equal(x$.label, c("four-wheel", "forward", "reverse"))
+
+  y <- get_guide_data(b2, "y")
+  expect_equal(y$y, rescale(seq(2.5, 10, by = 2.5), from = b1$y.range))
+})
+
 test_that("discrete scales can map to 2D structures", {
 
   p <- ggplot(mtcars, aes(disp, mpg, colour = factor(cyl))) +