%!

% t -> [ t^2-1, t^3-t]
/f { 1 dict begin
/t exch def
[
  t dup mul 1 sub
  t dup dup mul mul t sub
]
end } def

% a b N
/make-curve { 1 dict begin
/N exch def
/b exch def
/a exch def
/t a def
/dt b a sub N div def
t f aload pop moveto
N {
  /t t dt add def
  t f aload pop lineto
} repeat
end } def

% --- drawing --------------------------------------------

72 dup scale
1 72 div setlinewidth

4.25 5.5 translate

newpath
-3 -3 moveto
3 -3 lineto
3 3 lineto
-3 3 lineto
closepath
clip

gsave
0.7 setgray
newpath
-3 1 3 { /i exch def
i -3 moveto
i  3 lineto
-3 i moveto
 3 i lineto
} for
stroke
grestore

gsave
currentlinewidth 2 mul setlinewidth
newpath
-3 0 moveto
 3 0 lineto
 0 -3 moveto
 0 3 lineto
stroke
grestore

0.6 0 0.6 setrgbcolor
newpath
-2 2 32 make-curve 
stroke