Simple force directed graph drawing algorithm

spring.py

#!/usr/bin/env python

# Force-Directed Graph Drawing

import Tkinter
import random
import math

# d = [
#     [.0, .3, .3, .0],
#     [.3, .0, .3, .0],
#     [.3, .3, .0, .3],
#     [.0, .0, .3, .0]
# ]

d = []
nrows = 5
ncols = 5
for i in xrange(nrows * ncols):
    ci = i % ncols
    ri = i / ncols
    dr = []
    for j in xrange(nrows * ncols):
        cj = j % ncols
        rj = j / ncols
        if (ci == cj) and (ri==rj-1 or ri==rj+1) or (ri==rj and (ci==cj-1 or ci==cj+1)):
            dr.append(.1)
        else:
            dr.append(.0)
    d.append(dr)
# mass
alpha = 1.0
beta = .0001
k = 1.0
#damping
eta = .99
delta_t = .01

m = len(d)

root = Tkinter.Tk()

canvas = Tkinter.Canvas(root, width=500, height=500, background="yellow")
canvas.pack()

x = []
v = []
ids = []

def move_oval(i):
    newx = int(x[i][0] * 500)
    newy = int(x[i][1] * 500)
    canvas.coords(ids[i], newx - 5, newy - 5, newx + 5, newy + 5)

for i in xrange(m):
    xi = [random.random(), random.random()]
    x.append (xi)
    v.append ([0.0, 0.0])
    id = canvas.create_oval(245, 245, 255, 255, fill="red")
    ids.append(id)
    move_oval(i)

lids = []

def move_line(id, xi, xj):
    canvas.coords(id, int(xi[0] * 500), int(xi[1] * 500), int(xj[0] * 500), int(xj[1] * 500))

for i in xrange(m):
    for j in xrange(m):
        if d[i][j] != 0:
            id = canvas.create_line(0, 0, 0, 0)
            lids.append(id)
            move_line(id, x[i], x[j])

def Coulomb_force(xi, xj):
    dx = xj[0] - xi[0]
    dy = xj[1] - xi[1]
    ds2 = dx*dx + dy*dy
    ds = math.sqrt (ds2)
    ds3 = ds2*ds
    if ds3 == 0.0:
        const = 0
    else:
        const = beta / (ds2*ds)
    return [-const * dx, -const * dy]

def Hook_force(xi, xj, dij):
    dx = xj[0] - xi[0]
    dy = xj[1] - xi[1]
    ds = math.sqrt (dx*dx + dy*dy)
    dl = ds - dij
    const = k *  dl / ds
    return [const * dx, const * dy]


def move ():
    for i in xrange(m):
        Fx = 0.0
        Fy = 0.0
        for j in xrange(m):
            if j == 1:
                continue
            dij = d[i][j]
            Fij = 0.0
            if dij == 0.0:
                Fij = Coulomb_force(x[i], x[j])
            else:
                Fij = Hook_force(x[i], x[j], dij)
            Fx += Fij[0]
            Fy += Fij[1]
        v[i][0] = (v[i][0] + alpha * Fx * delta_t) * eta
        v[i][1] = (v[i][1] + alpha * Fy * delta_t) * eta


    for i in xrange(m):
        x[i][0] += v[i][0] * delta_t
        x[i][1] += v[i][1] * delta_t
        move_oval(i)

    li = 0
    for i in xrange(m):
        for j in xrange(m):
            if d[i][j] != 0:
                id = lids[li]
                move_line(id, x[i], x[j])
                li += 1

    root.after(1, move)

root.after(1, move)

root.mainloop()

looks like there is a bug on line 111, shouldn't that be an i not a 1? i wish there were more comments about what the 'd' matrix does. It is a 5x5 matrix, with either 0 or 0.1 in it; some kind of convolution kernel?