import numpy as np
import matplotlib.pyplot as plt
import pandas as pd

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

idx = pd.date_range("2020-01-01", periods=1000)
val = np.random.rand(1000)
s = pd.Series(val, idx)

g = s.groupby([s.index.year, s.index.month]).mean()

ax = g.plot()
ax.set_xticks(range(len(g)))
ax.set_xticklabels(["%s-%02d" % item for item in g.index.tolist()],
rotation=45, ha='center')

plt.show()