Principal components projection along the 2 principal eigenvectors (the 1^st and 2^nd axes) for the distributions of amino acid replacement scores in 12 matrices[1].

1. Mark S. Johnson and John P. Overington
   A Structural Basis for Sequence Comparisons: An Evaluation of Scoring Methodologies
   J. Mol. Biol. (1993) 233, 716-738.