graphing euler method

[scidam]

I slightly rewrote your code, you can adapt it for your needs.

import numpy as np
import matplotlib.pylab as plt

def volterra(P, K=1, r=1):
    return r * P * (1 - P / K)

def euler(fun, Po, h, N, **kwargs):
    acc = [Po]
    times = np.cumsum(np.r_[0, np.ones(N) * h])
    for _ in range(N):
        acc.append(acc[-1] + h * fun(acc[-1], K=K, r=r))  # for autonomous system's only
    return np.array(acc), times

K = 12
r = 0.43
Po = 1
N = 30;
step_values = [1, 0.5, 0.1]
fig = plt.figure()
ax = fig.add_subplot(111)
for h, c in zip(step_values, 'rgb'):
    Ps, Ts = euler(volterra, Po, h, N, K=K, r=r)
    ax.plot(Ts, Ps, c)

plt.show( )