Unicode minus

You can use the proper typesetting Unicode minus (see https://en.wikipedia.org/wiki/Plus_sign#Plus_sign) or the ASCII hyphen for minus, which some people prefer. The matplotlibrc param axes.unicode_minus controls the default behavior.

The default is to use the Unicode minus.

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

# Fixing random state for reproducibility
np.random.seed(19680801)


matplotlib.rcParams['axes.unicode_minus'] = False
fig, ax = plt.subplots()
ax.plot(10*np.random.randn(100), 10*np.random.randn(100), 'o')
ax.set_title('Using hyphen instead of Unicode minus')
plt.show()