Question

I want to write a function that calls both plot() and legend() and it would be ideal if the user could specify a number of additional arguments that are then passed through to either plot() or legend(). I know I can achieve this for one of the two functions using ...:

foo.plot <- function(x,y,...) {
    plot(x,y,...)
    legend("bottomleft", "bar", pch=1)
}

foo.plot(1,1, xaxt = "n")

This passes xaxt = "n" to plot. But is there a way for example to pass e.g. title = "legend" to the legend() call without prespecifying the arguments in the function header?

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Update from the accepted answer: I thought that VitoshKa’s way was the most elegant to accomplish what I wanted. However, there were some minor issues that I had to get around with until it worked as I wanted.

At first, I checked which of the parameters I want to pass to legend and which to plot. First step to this end was to see which arguments of legend are unique to legend and not part of plot and/or par:

legend.args <- names(formals(legend))
plot.args <- c(names(formals(plot.default)), names(par()))
dput(legend.args[!(legend.args %in% plot.args)])

I use dput() here, because the line plot.args <- c(names(formals(plot.default)), names(par())) always calls a new empty plot which I did not want. So, I used the output of dput in the following function.

Next, I had to deal with the overlapping arguments (get them via dput(largs.all[(largs.all %in% pargs.all)])). For some this was trivial (e.g., x, y) others get passed to both functions (e.g., pch). But, in my real application I even use other strategies (e.g., different variable names for adj, but not implemented in this example).

Finally, the do.call function had to be changed in two ways. First, the what part (i.e., called functions) needs to be a character (i.e., 'plot' instead of plot). And the argument list must be constructed slightly different.

foo.plot <- function(x,y,...) {
    leg.args.unique <- c("legend", "fill", "border", "angle", "density", "box.lwd", "box.lty", "box.col", "pt.bg", "pt.cex", "pt.lwd", "xjust", "yjust", "x.intersp", "y.intersp", "text.width", "text.col", "merge", "trace", "plot", "ncol", "horiz", "title", "inset", "title.col", "title.adj")
    leg.args.all <- c(leg.args.unique, "col", "lty", "lwd", "pch", "bty", "bg", "cex", "adj", "xpd")
    dots <- list(...)
    do.call('plot', c(list(x = x, y = x), dots[!(names(dots) %in% leg.args.unique)]))
    do.call('legend', c(list("bottomleft", "bar"), dots[names(dots) %in% leg.args.all]))
}


foo.plot(1,1,pch = 4, title = "legendary", ylim = c(0, 5))

In this example, pch is passed to both plot and legend, title is only passed to legend, and ylim only to plot.

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Update 2 based on a comment by Gavin Simpson (see also the comments at Vitoshka’s answer):
(i) That’s correct.
(ii) It can always be a character. But if you have a variable with the same name as the function, then you need to quote the function name in do.call:

min.plot <- function(x,y,plot=TRUE) if(plot == TRUE) do.call(plot, list(x = x, y = y))
min.plot(1,1)
Error in do.call(plot, list(x = x, y = y)) : 
  'what' must be a character string or a function

(iii) You can use c(x = 1, y = 1, list()) and it works fine. However, what I really did (not in the example I gave but in my real function) is the following: c(x = 1, y = 1, xlim = c(0, 2), list(bla='foo'))
Please compare this with: c(list(x = 1, y = 1, xlim = c(0, 2)), list(bla='foo'))
In the first case, the list contains two elements xlim, xlim1 and xlim2 (each a scalar), in the latter case the list has only xlim (which is vector of length 2, which is what I wanted).

So, you are right in all your points for my example. But, for my real function (with a lot more variables), I encountered these problems and wanted to document them here. Sorry for being imprecise.

Answer 1

An automatic way:

foo.plot <- function(x,y,...) {
    lnames <- names(formals(legend))
    pnames <- c(names(formals(plot.default)), names(par()))
    dots <- list(...)
    do.call('plot', c(list(x = x, y = x), dots[names(dots) %in% pnames]))
    do.call('legend', c("bottomleft", "bar", pch = 1, dots[names(dots) %in% lnames]))
}

pch must be filtered from the lnames to avoid duplication in the legend call in case the user supplies ‘pch’, but you got the idea.
Edited Jan 2012 by Carl W: “do.call” only works with the functions in quotes, as in the updates by Henrik. I edited it here to avoid future confusion.