matplotlib stacked plot not working as expected

Question

I am trying to use matplotlib's hist function to make stacked plots. Here is what my code block looks like:

a = np.array([5.2, 6.7, 8.9, 10.5, 4.2, 2.1])
b = np.array([3.1, 2.2, 9.9, 15.5, 3.8, 1.4])
fig,ax = plt.subplots(1,1,figsize=(7,7))
values = np.arange(0, 6, 1)

plt.hist(values, weights=a,bins=5, histtype='barstacked', label='a')
plt.hist(values, weights=b, bins=5, histtype='barstacked',label='b')
plt.legend()
plt.show()

I also tried histtype='bar' but that also doesn't seem to work. I must be missing something very obvious here, but the two hists are clearly not stacked.

Any help would be appreciated.

Answer 1

Try using bottom parameter with the output from first histogram, note the return, the n element, of plt.hist:

g = plt.hist(values, weights=a, bins=5, label='a')
plt.hist(values, weights=b, bins=5, label='b', bottom=g[0])

Output: