directlabels - utility.function - Positioning Method - only.unique.vals

Create a 1-row data.frame consisting of only the columns for which there is only 1 unique value.

only.unique.vals <- function(d,...){
  unique.vals <- lapply(d,unique)
  n.vals <- sapply(unique.vals,length)
  do.call(data.frame,unique.vals[n.vals==1])
}