De Grey (2018) found the first examples of unit-distance graphs with chromatic number 5, thus demonstrating
that the solution to the Hadwiger-Nelson problem
(i.e., the chromatic number of the plane) is at least 5. De Grey he was able to reduce
the size of his smallest example (after a correction) to the 1581-vertex de
Grey graph (de Grey 2018).
A few days after the original preprint was published, Mixon (2018) constructed a similar 1585-vertex graph, the removal of 8 vertices from which led to an even smaller 1577-vertex graph. This work terms these graphs the Mixon graphs, illustrated above.