Basic function plotter

fplot.py

import pygame, sys
from pygame.locals import *
from math import floor
import math

WIDTH, HEIGHT = 320, 200

pygame.init()
pygame.display.set_mode((WIDTH, HEIGHT))
pygame.display.set_caption('Function plotter')

screen = pygame.display.get_surface()

# Scales values in [a, b] to [c, d].
class Scaler:
  def __init__(self, a, b, c, d):
    self.quot = float(d - c) / float(b - a)
    self.term = -a * self.quot + c
  
  def scale(self, x):
    return x * self.quot + self.term

class Window:
  def __init__(self, scaleX, scaleY):
    self.scaleX = scaleX
    self.scaleY = scaleY
  
  def scale(self, x, y):
    return (self.scaleX.scale(x), self.scaleY.scale(y)) 

def handle_input(events):
  for event in events:
    if event.type == QUIT:
      sys.exit(0)

def stepper(a, b, delta):
  x = a
  
  while x <= b:
    yield x
    x += delta

def get_coords(x, win, func):
  px, py = win.scale(x, func(x))
  return (px, HEIGHT - py)

window = Window(Scaler(-10, 10, 0, WIDTH), Scaler(-100, 100, 0, HEIGHT))

while True:
  screen.fill(Color(255, 255, 255, 0))
  pygame.draw.line(screen, Color(0, 0, 0, 0), (WIDTH/2, 0), (WIDTH/2, HEIGHT))
  pygame.draw.line(screen, Color(0, 0, 0, 0), (0, HEIGHT/2), (WIDTH, HEIGHT/2))
  
  f = eval('lambda x: ' + raw_input('f(x) = '))
  
  s = stepper(-10, 10, 0.1)
  last = get_coords(s.next(), window, f)
  for x in s:
    px, py = get_coords(x, window, f)
    if py >= 0 and py < HEIGHT:
      pygame.draw.line(screen, Color(255, 0, 0, 0), last, (px, py))
    last = (px, py)
  
  pygame.display.flip()