%matplotlib inline
import matplotlib.pyplot as plt

fig = plt.figure()

ax = fig.add_subplot(1, 1, 1)
ax.set_aspect(1)

c = plt.Circle((0.5, 0.5), 0.2, color="r", alpha=0.5)
ax.add_artist(c)
ax.text(0.5, 0.5, "hello world!", ha="center", va="center")