[R] plots: layout + subtitles

[Marie-Pierre Sylvestre]

Hello,

I want to create a figure that consists of a collection of 16 graphs on 
4 rows.  I am using

nf <- layout(matrix(seq(1,16), 4,4, byrow=TRUE), respect=TRUE)
boxplot(...

to create the layout of my 16 graphs. It works really well. However, I'd 
like to add sub-titles that would apply to each row of 4 graphs. More 
specifically, I'd like to have something like:

subtitle 1 (centered)
graph 1 graph 2 graph 3 graph 4
subtitle 2 (centered)
graph 5 graph 6 graph 7 graph 8
subtitle 3 (centered)
graph 9 graph 10 graph 11 graph 12
subtitle 4 (centered)
graph 13 graph 14 graph 15 graph 16

in the same figure, to be saved as a ps file.

Can somebody help?
I hope this question is not redundant, but I have not found information 
on this on the R mailing lists.

I use R 2.3.1 on Linux FC5.

Thank you in advance.

Marie-Pierre Sylvestre
PhD student, McGill University