a(0) = 1 from the complete graph K_4.
a(1) = 1 from the utility graph K_{3,3}.
a(2) = 2 from the Petersen graph (and 1 other).
a(3) = 8 from the Heawood graph, GP(7,2) (and 6 others).
a(4) = 2 from the MoebiusKantor and 8crossed prism graphs.
a(5) = 2 from the Pappus graph (and 1 other).
a(6) = 3 from the Desargues graph (and 2 others based on 10_3 configurations).
a(7) = 4 from the 7crossing graphs (4 in total).
a(8) = 3 from the McGee and Nauru graphs (and 1 other).
a(9) >= 3 from GP(13,5), the Coxeter1, and McGee+1 graphs (and unknown others).  Eric W. Weisstein, Apr 12 2019
a(10) >= 2 from the Levi1 (=McGee+2) graph and graph from 2019 Clancy et al. preprint (and unknown others).  Eric W. Weisstein, Apr 12 2019
a(11) >= 1 from the Coxeter graph (and unknown others).
a(13) >= 1 from the Levi graph (and unknown others).
