import numpy as np
import datetime
from matplotlib import pyplot as plt

plt.rcParams["figure.figsize"] = [7.50, 3.50]
plt.rcParams["figure.autolayout"] = True

x = [datetime.datetime(2021, 1, 1, 0, 0),
     datetime.datetime(2021, 2, 1, 0, 0),
     datetime.datetime(2021, 3, 1, 0, 0),
     ]

y = np.cos(np.arange(3) * 2)
plt.bar(x, y, width=[(x[j+1]-x[j]).days for j in range(len(x)-1)] + [30])

plt.show()