Treat data as list during position transformation (#3002)

* Treat input as list

* Add bullet to vignette

* Make note about changes related to performance

Author: thomasp85

R/position-.r

@@ -75,6 +75,9 @@ Position <- ggproto("Position",
 #' @keywords internal
 #' @export
 transform_position <- function(df, trans_x = NULL, trans_y = NULL, ...) {
+  # Treat df as list during transformation for faster set/get
+  oldclass <- class(df)
+  df <- unclass(df)
   scales <- aes_to_scale(names(df))
 
   if (!is.null(trans_x)) {
@@ -84,5 +87,7 @@ transform_position <- function(df, trans_x = NULL, trans_y = NULL, ...) {
     df[scales == "y"] <- lapply(df[scales == "y"], trans_y, ...)
   }
 
+  class(df) <- oldclass
+
   df
 }

vignettes/profiling.Rmd

@@ -67,3 +67,6 @@ are summarised below:
   and will just lead to slower code. The `data.frame()` call is now only used
   when dealing with output from other packages where the extra safety is a 
   benefit.
+- **Speed up position transformation** The `transform_position` helper was 
+  unreasonably slow due to the slowness of getting and assigning columns in 
+  data.frame. The input is now treated as a list during transformation.