Allow setting one of the limits of scale

Implements setting of only one limit of a continuous scale by specifying
NA for the limit which you don't want to set.

Author: jimhester

R/limits.r

@@ -47,7 +47,7 @@ limits <- function(lims, var) UseMethod("limits")
 #' @export
 limits.numeric <- function(lims, var) {
   stopifnot(length(lims) == 2)
-  if (lims[1] > lims[2]) {
+  if (sum(is.na(lims)) == 0 &lims[1] > lims[2]) {
     trans <- "reverse"
   } else {
     trans <- "identity"

R/scale-.r

@@ -298,7 +298,9 @@ scale_limits <- function(scale) {
 
 #' @export
 scale_limits.default <- function(scale) {
-  scale$limits %||% scale$range$range
+  if(!is.null(scale$limits)) 
+    ifelse(!is.na(scale$limits),scale$limits,scale$range$range) 
+  else scale$range$range
 }
 
 # @kohske