Don't want original data.table to be modified when passed to a function

Question

I am a fan of data.table, as of writing re-usable functions for all current and future needs.

Here\'s a challenge I run into while working on the answer

Answer 1

Just leaving out the := function seemed to succeed. Of course I wrapped the ggplot value in print(.) as would be standard practice when working inside a function and wanting output.:

plotYofX <- function(.dt,x,y) {
  dt <- .dt
  dt[,  lapply(.SD, function(x) {as.numeric(x)}), .SDcols = c(x,y)]
  print( ggplot(dt) + geom_step(aes(x=get(names(dt)[x]), y=get(names(dt)[y]))) + labs(x=names(dt)[x], y=names(dt)[y]) )
}

> png(); plotYofX(dtDiamonds,1,2); dev.off()
quartz 
     2 
>  dtDiamonds
   carat     cut color
1:  0.21 Premium     E
2:  0.23    Good     E
3:  0.29 Premium     I
4:  0.31    Good     J