How to create accumulated bar graph using Matplotlib

import matplotlib
matplotlib.use('Agg')
import matplotlib.pyplot as plt
import numpy as np

fig = plt.figure()
ax1 = fig.add_subplot(1, 1, 1)
p0 = ax1.bar(range(20), range(20), color='blue')
p1 = ax1.bar(range(20), (2*np.array(range(20))).tolist(), color='blue')
ax1.legend((p0[0], p1[0]), ('bar1', 'bar2'))
ax1.grid()
fig.savefig('aaa.png')