Heule graphs are a set of unit-distance graphs with chromatic number five derived by Marijn Heule in April-May 2018 from the 1581-vertex de Grey Graph (Heule 2018). As of May 2018, they provide the smallest known examples that establish the solution to the Hadwiger-Nelson problem (i.e., the chromatic number of the plane) as 5, 6, or 7. These graphs were discovered in the days and weeks following publication of the de Grey graph and are summarized in the following table. The first few are...