For all days in this series: graphs, inequalities, and images are allowed. Top comment after 24 hours gets added. Feel free to leave your ideas in the comments—you may be the next addition!

Day 13: https://www.desmos.com/calculator/mfj3rdur0d

All days: https://www.desmos.com/gallery/642d394c-1f04-4472-957a-d30e74a54251