change certain squares in a seaborn heatmap

Question

Say I have a heatmap that looks like this (axes are trimmed off):

I want to be able to alter certain squares to denote statistical significance. I know that I could

Answer 1

You could plot twice, applying a mask to the cells you do not want to emphasize the second time:

import numpy as np
import seaborn as sns
x = np.random.randn(10, 10)
sns.heatmap(x, annot=True)
sns.heatmap(x, mask=x < 1, cbar=False,
            annot=True, annot_kws={"weight": "bold"})