help with formula and data= argument

Sorry for posting this twice, but I still have not solved this problem
and am hoping for some assistance.



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 code illustrates (I know that
plotrix::thigmophobe.labels() does what this function does) my problem
...





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" when using the formula 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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