[R] draw partition lines

[questions?]

I am using par(mfrow(4,2)...) to plot multiple graphs in one frame.
However, the plots naturally partitioned into two sets, the first two
rows and the 3rd and 4th row are slightly different,
I want them to be in one frame, but I also want to draw a line (or
something to make top two rows/bottom two rows look slightly
different) in between them?

Any suggestions?

Thanks