annotate を使う。
例
import numpy as np
from matplotlib import pyplot as plt
x = np.linspace(-10,10,100)
y = np.sin(x)
fig, ax = plt.subplots()
ax.scatter(x,y, s=100, alpha=0.3,label="sin")
ax.annotate(text="sine function",xy=(0.3,0.5),color='blue')
plt.show()
結果
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