substitute, eval, quote and functions

I think David was
looking for something like

function(data, x, y)

        eval(substitute(plot(x,y)), data)

This was _exactly_ what i was looking for, after having spent quite some 
time reading the language definition and some example code such as 
subset.data.frame.  Thanks!

So now I've got my function that plots two dependent variables in 
different coulors and/or point types.  It even makes legends, although 
the positioning isn't great.  It can probably do with some code cleaning 
up, but hey, I'm new.  There might even be a function that does this 
already, but I haven't found it yet.

---david

## Plots colums `xcol' vs. `ycol' in colors accortding to levels `ccol',
## and point types according to `pcol'.
##
## optionally first select rows under condition `cond'

plotcol <- function(data, xcol, ycol, ccol=NULL, pcol=NULL, cond=TRUE) {
  ## extract relevant data
  s <- eval(substitute(subset(data, cond, c(xcol,ycol,ccol,pcol))), data)
  ## help to place the legends
  xrange <- range(s[,1], na.rm=TRUE)
  yrange <- range(s[,2], na.rm=TRUE)
  ## colour
  if (missing(ccol)) col=1
  else {
    ccol <- eval(substitute(ccol), s)   # replace name with data
    col <- as.numeric(ccol)                # make colors
  }
  ## point type
  if (missing(pcol)) pch=1
  else {
    pcol <- eval(substitute(pcol), s)
    pch=as.numeric(pcol)
  }
  ## make the plot
  plot(s[,1:2], col=col, pch=pch)
  ## and fix the legends
  if (length(col)>1) {
    size <- legend(xrange[1], yrange[2], legend=levels(ccol),
                   col=1:nlevels(ccol), pch=1, lty=1)
    y <- yrange[2]-size$rect$h          # calculate pos of next legend
  } else y <- yrange[2]
  if (length(pch)>1)
    legend(xrange[1], y, legend=levels(pcol), pch=1:nlevels(pcol))
}