Multiple plots with single box

Hello,

I am trying to display some harmonic functions in a plot. The kind of
display I have in mind is like the one that cn be obtained by a call
to plot.ts with plot.type = "multiple". The only difference is that I
want a single box containing all the plots instead of one box per
plot. I thought box(which = "outer") would have done the job, but it
didn't. 

Below is the code I have used so far. (R 2.5.1, I know, I know...)

Any help is greatly appreciated. 

Thank you in advance,
Giovanni

=================
### Plot harmonic functions
n <- 6 # even
omega <- 2 * pi / n

par(mfrow = c(n - 1, 1), mar = c(0, 5.1, 0, 5.1), oma = c(3, 1, 2, 1))
for (i in 1:(n/2 - 1)) {
    curve(cos(x * i * omega), 0, n, ylim = c(-1.1, 1.1), ylab = "", axes = FALSE)
    points(1:n, cos(i * omega * 1:n))
    axis(2); abline(h = 0, col = "lightgrey")
    curve(sin(x * i * omega), 0, n, ylim = c(-1.1, 1.1), ylab = "", axes = FALSE)
    points(1:n, sin(i * omega * 1:n))
    axis(4); abline(h = 0, col = "lightgrey")
}
curve(cos(x * (n/2) * omega), 0, n, ylim = c(-1.1, 1.1), ylab = "", axes = FALSE)
points(1:n, rep(c(-1,1), n/2))
axis(1); axis(2); abline(h = 0, col = "lightgrey")