Create parallelogram (no -Sj)

Hi,
I’d like to draw a parallelogram using a file with this format :

xa1 xa2 xa3 xa4 ya1 ya2 ya3 ya4    # polygon 1
xb1 xb2 xb3 xb4 yb1 yb2 yb3 yb4    # polygon 2
...
xz1 xz2 xz3 xz4 yz1 yz2 yz3 yz4     # polygon N

The closest I can find is using plot with -Sr+s, but it’s not directional like -Sj

Any idea ?

Of course, I still have the solution to extensively use awk
edit : which I have now implemented. The question remain though, I’m curious for next time.