[R] Unequal sized three plots in a window

[Rangrej, Jagadish]

Dear all:

I want to divide the graphing window in to three plots such that two plots
are on the top and one at the bottom. The upper two plots are of equal sizes
that cover 50% of the Window, third plot spans the lower half portion of the
Window completely.

How do I do that, I tried layout and other options but they seem to have the
matrix options which does not solve my problem.

Any help or pointer will be greatly appreciated.

Thank you,
-Jag