[R] multiple x-axis labels on plot

[t.d. kaplan]

hi,

how can i set multiple x-axis labels on a plot? in effect, i want
to set 2 x-axis labels and values on the tick marks. is it possible
to have one running on the bottom (as is the default) and another
running across the top of a plot?

like:

xlab-1
---------
|       |
|       |
---------
xlab-0


thanks.