Fake Polygraph Python Pygame

import pygame
import time

# define display surface
W, H = 1280, 720

# initialise display
pygame.init()
CLOCK = pygame.time.Clock()
DS = pygame.display.set_mode((W, H))
FPS = 500.00
MSPF = 1.00 / FPS

# define some colors
BLACK = (0, 0, 0, 255)
WHITE = (255, 255, 255, 255)

dot_size = 5
dot_x = W - dot_size
del_y = 0

exit = False

startTime = time.time()
# main loop
while True:
    for event in pygame.event.get():
        if event.type == pygame.QUIT or (event.type == pygame.KEYDOWN and event.key == pygame.K_ESCAPE):
            exit = True
    if exit: break


    my = pygame.mouse.get_pos()[1]
    pygame.draw.line(DS, WHITE, (dot_x, del_y), (dot_x, my), 5) #circle(DS, WHITE, (dot_x, my), dot_size, 0)
    del_y = my

    testTime = time.time()
    if testTime - startTime >= MSPF:
        startTime += MSPF
        DS.blit(DS, (0, 0), (1, 0, W - 1, H))
        pygame.display.update()

pygame.quit()