%!

200 200 translate
/S 200 def
S dup scale
1 S div setlinewidth

% what is the function?
/f { 1 dict begin
  /x exch def
  x x mul 
  3 x 2 mul sub
  mul 
end } def

newpath
0 0 moveto
1 0 lineto
1 1 lineto
0 1 lineto
closepath
stroke

gsave
currentlinewidth 4 mul setlinewidth
1 0 0 setrgbcolor
/N 20 def
/x 0 def
/dx 1 N div def
newpath
x x f moveto
1 1 N { /i exch def
/x dx i mul def
x x f lineto
} for
stroke
grestore