math graph

#!/usr/bin/python
from tkinter import *
from math import *

resy=480
resx=int((resy/9)*16)

window = Tk()
canvas = Canvas(window, width=resx, height=resy, bg="#000000")
canvas.pack()
img = PhotoImage(width=resx, height=resy)
item = canvas.create_image((int((resx/2)), int((resy/2))), image=img, state="normal")

txt = "cos(radians(x))*100"
def strt():
    global resx
    global img
    global canvas

    x=resx
    while x>=0:
        img.put("#9A1409", (x,int((resy/2))))
        x=x-1

    x=resx
    while x>=0:
        img.put("#9A1409", (int((resx/2)),x))
        x=x-1

w1 = Scale(window, from_=-int(resx/2), to=int(resx/2),orient=HORIZONTAL,resolution=0.5, length=resx+25)
w1.pack()

var=StringVar()
var.set(txt)
R=Label(window, textvariable=var, height=1, width=50)
R.pack()

def callback():
    global txt
    global x
    global canvas
    global item
    global img
    global item

    canvas.delete(item)
    img = PhotoImage(width=resx, height=resy)
    item = canvas.create_image((int((resx/2)), int((resy/2))), image=img, state="normal")

    strt()
    x=-int((resx/2))
    txt = e.get()

x=-int((resx/2))

def text(txt):
    global var
    x = w1.get()
    exec("var.set("+txt+")")

def update():
    global x
    global var
    global txt
    global img

    if x>=-int((resx/2)) and x<=int((resx/2)):
        y=eval(str(txt))
        z=int(y)
        if y < int((resy/2)) and y > -int((resy/2)):
            img.put("#00ff00", (int(x)+int((resx/2)),int((resy/2))-(z)))

    if x<int(resx/2):
        x=x+0.1

    text(txt)

    window.after(1,update)

e = Entry(window)
e.pack()

b = Button(window, text = "OK", width = 10, command = callback)
b.pack()
strt()
window.after(1,update)
window.mainloop()