如何在Matplotlib中设置月末的刻度线

import matplotlib.pyplot as plt
import matplotlib.dates as mdates
from datetime import datetime

dates = [
    datetime(2022, 1, 1),
    datetime(2022, 1, 10),
    datetime(2022, 1, 15),
    datetime(2022, 1, 20),
    datetime(2022, 2, 1),
    datetime(2022, 2, 10),
    datetime(2022, 2, 15),
    datetime(2022, 2, 28),
    datetime(2022, 3, 1),
    datetime(2022, 3, 10),
    datetime(2022, 3, 15),
    datetime(2022, 3, 20)
]

values = [10, 20, 15, 30, 25, 35, 40, 30, 20, 15, 25, 35]

fig, ax = plt.subplots()
dates = mdates.date2num(dates)

ax.plot_date(dates, values, '-')

ax.xaxis.set_major_locator(mdates.MonthLocator())
ax.xaxis.set_minor_locator(mdates.WeekdayLocator())
ax.xaxis.set_major_formatter(mdates.DateFormatter('%b %Y'))

ax.grid(True)
plt.title('Monthly Data')

plt.show()