First, you need to eat people. Primary step to make the lib run. Else, ERR:LOLI occurs then pressing PRGM causes a RAM Clear.
Secondly,
x1->A
x2->B
y1->C
y2->D
Sub(SQ)
Lbl SQ
Line(A,C,A,D
Line(A,D,B,D
Line(B,D,B,C
Line(B,C,A,C
DispGraph
Return
I tested it on my TI-Nspire and it seems to work.
THis should draw a rectangle.
There might be more efficient ways to do this, though, but this is the first idea I had in my mind.