Change minor_breaks interface

To pass in both limits and major breaks. This will make it much easier to create useful families of minor break functions in scales.

Fixes #3583

Author: hadley

R/scale-.r

@@ -24,7 +24,8 @@
 #'   - `waiver()` for the default breaks (one minor break between
 #'     each major break)
 #'   - A numeric vector of positions
-#'   - A function that given the limits returns a vector of minor breaks.
+#'   - A function passed the limits and major breaks, which should return a
+#'     vector of minor breaks.
 #' @param labels One of:
 #'   - `NULL` for no labels
 #'   - `waiver()` for the default labels computed by the
@@ -613,8 +614,15 @@ ScaleContinuous <- ggproto("ScaleContinuous", Scale,
         breaks <- self$trans$minor_breaks(b, limits, n)
       }
     } else if (is.function(self$minor_breaks)) {
-      # Find breaks in data space, and convert to numeric
-      breaks <- self$minor_breaks(self$trans$inverse(limits))
+      breaks_fun <- self$minor_breaks
+
+      if (length(formals(breaks_fun)) == 1) {
+        # Old API just gets limits
+        breaks <- breaks_fun(self$trans$inverse(limits))
+      } else {
+        # New API gets limits and breaks
+        breaks <- breaks_fun(self$trans$inverse(limits), b)
+      }
       breaks <- self$trans$transform(breaks)
     } else {
       breaks <- self$trans$transform(self$minor_breaks)