Complementary-Color, Two-Variable Maps

Complementary color coding is used in conjunction with nontraditional legend arrangements to create two-variable maps. The logic of a complementary color-coding scheme is simpler and easier for the map user to comprehend than the spectrally encoded scheme used to produce most of the two-variable maps published to date. In addition to the complementary color-coding scheme, an unclassed legend based on a line of best fit and a legend arrangement based on the binormal distribution are used to represent more accurately the structure of the bivariate scatter diagram. These legends avoid many of the statistical distortions introduced by the rectangular legend currently being used to produce two-variable maps. Although the unclassed two-variable map presents a true graphic correlation, the large number of color tones contained in the map confounds the map user. The two-variable map based on the binormal distribution combines the rectangular legend and an equiprobability ellipse to represent the legend diagonal. This appears to be a good compromise for the production of complementary-color, two-variable maps.