[R] Working with and without formula in function

[dogle]

I am attempting to write a function that is flexible enough to respond to the
user providing a formula (with a data= argument) or not (similar to
plot(x,y) versus plot(y~x,data=data)).  I have found a method to work with
this in a simple case but am having trouble determining how to "find" a
variable from within the data= argument that is not part of the formula. 
The following illustrates my problem ...

# an illustrative function -- I know that plotrix::thigmophobe.labels() does
what this function does
myplot <- function(x,y=NULL,data=NULL,label=NULL) {
  if (class(x)=="formula") {
    mf <- model.frame(x,data=data)
    x <- mf[,2]
    y <- mf[,1]
  } 
  if (is.null(y)) stop("Y-axis variable is missing")
  plot(x,y)
  if (!is.null(label)) text(x,y,label)
}

# dummy data
df <-
data.frame(x=runif(10),y=runif(10),grp=factor(rep(c("Yes","No"),each=5)))

# both calls work as expected
with(df,myplot(x,y))
myplot(y~x,data=df)

# only first works as I would hope
with(df,myplot(x,y,label=grp))
myplot(y~x,data=df,label=grp)

# this works but is clumsy
myplot(y~x,data=df,label=df$grp)


Any help with how to make this function recognize the "grp" variable in "df"
without having to type "df$grp" when supplying it to the "label=" argument
would be greatly appreciated.  Thank you in advance.
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