Matplotlib fill_between shows a sliver of white when filing from zero

Question

Here is my code:

x = np.arange(-10, 10, 0.1)
y = np.cos(x)

f = plt.figure()
ax = f.add_subplot(1, 1, 1)
ax.spines[\'left\'].set_position(\'center\')
ax.spines[\'