R help - Aug 2007 - 2nd label category on x axis

i'm plotting time-series and label the x-axis as follows:

r <- as.POSIXct(round(range(p1$time), "month"))

plot(p1$time,p1$ p1, type="l", xaxt="n")

axis.POSIXct(1, at=seq(r[1], r[2], by="month"),
format="%Y.%m")

what I want do do now, is a second label for the x-axis, that stands lower and
indicates the years.

I don't know how to proceed with all the possibilities in axis, par and plot

thanks for your help