[R] plotting fit from ols( rcs() )

[x]

I have: f <- ols( y1 ~ (rcs(x1,3) + rcs(x2,3) + rcs(x1,3) %ia% rcs(x2,3) ) )

Then I do: plot(f)

This plots y1 against x2 for a given value of x1.

Is there a way to make it *also* plot y1 against x1 for a given value of x2?

If not, I guess I can do coplot(y1 ~ x1 | x2).

Thanks in advance,
sp