Python 2.7, Tkinter, make a point "trace" a waveform - python-2.7

Question

I'm trying to make a widget that demonstrates the relationship between a sine wave and it's phasor diagram. I want a pointer that follows the user's mouse movements but instead of freely moving around the page, I want it to stick to the waveform I've plotted and only follow the co-ordinates that make up that line. Kind of like a slider but along a sine wave! Any suggestions? Thanks. Here's what I've got so far;
import math
import Tkinter as tk
from Tkinter import PhotoImage, Canvas
from PIL import Image, ImageTk
sinwidget = tk.Tk()
main_canvas = tk.Canvas(sinwidget, width = 400, height = 400, bg= "white")
main_canvas.pack()
def callback(event):
canvas = event.widget
x = canvas.canvasx(event.x)
y = canvas.canvasy(event.y)
print canvas.find_closest(x, y)
draw(event.x, event.y)
def draw(x, y):
box.coords(pointer, x-20, y-20, x+20, y+20)
def exit_():
sinwidget.destroy()
box = main_canvas
box.bind('<Motion>', callback)
box.pack()
pointer = box.create_rectangle(0.2,0.2,0.2,0.2)
wavelength = 360
height = 400
center = height//2
degree = 1
increment = 0.0175
amplitude = -80
sin = []
for x in range(360):
sin.append(x * degree)
sin.append(int(math.sin(x * increment) * amplitude) + center)
sinwave = main_canvas.create_line(sin, fill="red", width=2.0)
xaxis = main_canvas.create_line(0, center, wavelength, center, fill="black",
width=3.0)
yaxis = main_canvas.create_line(2, 100, 2, 300, fill="black", width=3.0)
exit_button = tk.Button(sinwidget, text = "exit", command = exit_)
exit_button.pack()
sinwidget.mainloop()
Thanks for any help you can offer!