title: Change certain squares in a seaborn heat map
body: <p>Say I have a heat map that looks like this (axes are trimmed off):</p>
  <p><a href="https://i.stack.imgur.com/GI9t6.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/GI9t6.png" alt="Enter image description here" /></a></p>
  <p>I want to be able to alter certain squares to denote statistical significance. I know that I could mask out the squares that are not statistically significant, but I still want to retain that information (and not set the values to zero). Options for doing this include 1) making the text on certain squares bold, 2) adding a hatch-like functionality so that certain squares have stippling, or 3) adding a symbol to certain squares.</p>
  <p>What should I do?</p>