The search for rules for effective graphical display, whether for the purpose of communication, exploration, or reconstitution, has been hampered by the lack of a cohesive body of experimental evidence regarding the parameters of efficacious graphical display. To some extent existing evidence is diverse because of the lack of a coordinating theoretical structure and an allied unified graphical vocabulary. In this article we use the rudiments of Berlin's theory of graphics and his graphical semiology. We also present two experimental studies of Two-Variable Color Maps, which we offer as examples of how questions about the efficacy of a graphical form can be addressed.