10 setxunit
7.5 setborder
-13.5 13.5 setxrange
-13.5 13.5 setyrange
jpegmode
/a 1.5 def
/b 2.5 def
/c 3  def
/resolution 500 def
      
2 2 settkstep
1 1 setsubtkstep
2 2 setmkstep
ticks
traceaxes
axesarrow
unites
/X {settvar
    a b add t Cos mul c t 1 a b div add mul Cos mul sub
} def
/Y {settvar
    a b add t Sin mul c t 1 a b div add mul Sin mul sub
} def
gsave
   .75 setlinewidth
   continu
   bleu
  0  20 pi mul {X} {Y} Courbeparam
grestore