Construct time_trans with the given timezone (#4429)

Author: thomasp85

NEWS.md

@@ -1,5 +1,8 @@
 # ggplot2 (development version)
 
+* Fix bug in `scale_[x|y]_datetime()` where a specified timezone would be 
+  ignored by the scale (@thomasp85, #4007)
+  
 * Using `theme(aspect.ratio = ...)` together with free space in `facet_grid()`
   now crrectly throws an error (@thomasp85, #3834)
 

R/scale-date.r

@@ -314,6 +314,11 @@ datetime_scale <- function(aesthetics, trans, palette,
     scale_class <- ScaleContinuous
   }
 
+  trans <- switch(trans,
+    date = date_trans(),
+    time = time_trans(timezone)
+  )
+
   sc <- continuous_scale(
     aesthetics,
     name,