Anchored Box01

import matplotlib.pyplot as plt
from matplotlib.offsetbox import AnchoredText


fig, ax = plt.subplots(figsize=(3, 3))

at = AnchoredText("Figure 1a",
                  prop=dict(size=15), frameon=True, loc=2)
at.patch.set_boxstyle("round,pad=0.,rounding_size=0.2")
ax.add_artist(at)

plt.show()