Anyway, you need one coordinate axis for each unknown variable. Since you have 4 unknowns you will need 4 coordinate axes. As this is one more dimension than we live in, you'll have a little trouble finding the graph paper!
Going back to the other post what the teacher was suggesting was to convert the system to
x + y = 7.11
x*y = 7.11
and solving THIS system graphically. (But as I showed in the other post this doesn't give a real solution for all a, b, c, and d.)