[R] still trying to wrap xyplot - ignore previous

[Zack Weinberg]

As you may not be surprised to hear, no sooner did I post the previous
message than I realized I had a really dumb mistake.  I've now gotten
a bit farther but am still stuck.  New code:

graph <- function (x, data, groups, xlab) {
  pg <- function(x, y, group.number, ...) fnord
  body(pg) <- substitute({
    panel.xyplot(x, y, ..., group.number=group.number)
    panel.text(2, unique(y[x==2]),
               levels(G)[group.number],
               pos=4, cex=0.5)
  }, list(G=eval(substitute(groups), data, parent.frame())))

  print(xyplot(x, data=data, groups=substitute(groups),
               type='l',
               ylab=list(cex=1.1, label='Mean RT (ms)'),
               xlab=list(cex=1.1, label=xlab),
               scales=list(
                 x=list(alternating=c(1,1), tck=c(1,0)),
                 y=list(alternating=c(1,0))
                 ),
               panel=panel.superpose,
               panel.groups=pg
              ))
}

Questions:
1) The "groups=substitute(groups)" bit (in the call to xyplot) still
doesn't work.  As far as I can tell, xyplot wants the *symbol* which
is the name of the factor (in the data frame) to group by.
The above seems to wind up passing it the symbol "groups", which
causes the prepanel function to barf.  I have not been able to find
any way to evaluate one layer of "groups" to get me the symbol passed
in, rather than the value of that symbol.  Am I right?  How do I give
it what it wants?

2) Why do I have to do that stupid dance with replacing the body of
pg?  The documentation leads me to believe this is a lexically scoped
language, shouldn't it be able to pick G out of the enclosing frame?

zw