25 setxunit

-4 4 setxrange
-4 6 setyrange
1 1 settkstep
.5 .5 setsubtkstep
2 2 setmkstep
ticks
%marks
traceaxes
axesarrow
/f {setxvar
    x 3 exp 3 div x add 0.5 sub
} def
/g {setxvar
    x 3 exp 3 div x add 1 add
} def
/ff {setxvar
    x 3 exp 3 div x add 2 add
} def
/gg {setxvar
    x 3 exp 3 div x add 4 add
} def
gsave
   noir
   1.75 setlinewidth
   continu
   {f} courbe
grestore
gsave
   noir
   1.75 setlinewidth
   continu
   {g} courbe
grestore
gsave
   noir
   1.75 setlinewidth
   continu
   {ff} courbe
grestore
gsave
   noir
   1.75 setlinewidth
   continu
   {gg} courbe
grestore