Vertically Stacked Bar Graph

Let’s assume we have the following code snippet:

import matplotlib.pyplot as plt
import numpy as np
import pandas as pd
np.random.seed(0)
df = pd.DataFrame(data={'1':np.random.randint(0, 100, 10),
                        '2':np.random.randint(0, 100, 10)})

fig, ax = plt.subplots(2, 1, figsize=(15,18))                       
ax[0].bar(df.index.values, df['1'])
ax[0].bar(df.index.values, df['2'], _____?_____)
ax[0].legend(['1', '2'])
plt.show()

What should be placed in the above __?__ to make the df[‘2’] bars appear above the bars of df[‘1’] in a vertically stacked format in the barplot, similar to the image below?
Vertically Stacked Bargraph