controlliing a fill pattern in matplotlib

1import pandas as pd 
2import numpy as np
3
4n = 8
5a = np.random.random(n)
6b = np.random.random(n)
7x = np.arange(n)
8
9plt.bar(x,a, color='w', hatch='x')
10plt.bar(x,a+b, bottom = a, color = 'w', hatch = '/')
11plt.show()