20 SCREEN 12
30 LINE (320, 0)-(320, 480), 4
40 LINE (0, 240)-(640, 240), 4
80 FOR x = -320 TO 320



100 PSET (320 + x, 240 - 1 / 5000 * (x ^ 3)), 2
110 NEXT x

120 IF x = 400 THEN 200

130 GOTO 100

200 END