Round down breaks in scale_date to avoid misalignment (#4430)

Author: thomasp85

NEWS.md

@@ -1,5 +1,8 @@
 # ggplot2 (development version)
 
+* Fix bug in `scale_[x|y]_date()` where custom breaks functions that resulted in
+  fracional dates would get misaligned (@thomasp85, #3965)
+  
 * Fix bug in `scale_[x|y]_datetime()` where a specified timezone would be 
   ignored by the scale (@thomasp85, #4007)
 

R/scale-date.r

@@ -387,6 +387,14 @@ ScaleContinuousDate <- ggproto("ScaleContinuousDate", ScaleContinuous,
   map = function(self, x, limits = self$get_limits()) {
     self$oob(x, limits)
   },
+  get_breaks = function(self, limits = self$get_limits()) {
+    breaks <- ggproto_parent(ScaleContinuous, self)$get_breaks(limits)
+    if (is.null(breaks)) {
+      return(NULL)
+    }
+    breaks <- floor(breaks)
+    breaks[breaks >= limits[1] & breaks <= limits[2]]
+  },
   break_info = function(self, range = NULL) {
     breaks <- ggproto_parent(ScaleContinuous, self)$break_info(range)
     if (!(is.waive(self$secondary.axis) || self$secondary.axis$empty())) {