2 plots sharing axis / combining factors

- How could I convert 2 factor variables in a single factor variable
which is the combination of the other 2.
Example:
    lith: a factor 2 levels "ca", "ma"
    sp: a factor with 2 levels, "ph", "qi"
and I'd like to obtain:
    splith: a factor with 4 levels: phma, phca, qima, qica

strata(lith,sp) or interaction(lith,sp)

	-thomas


-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-
r-help mailing list -- Read http://www.ci.tuwien.ac.at/~hornik/R/R-FAQ.html
Send "info", "help", or "[un]subscribe"
(in the "body", not the subject !)  To: r-help-request at stat.math.ethz.ch
_._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._