They're just trying to illustrate 7 dimensional data. There are 67 2-dimensional comparisons, scatterplots, 67/2 if you eliminate ones made redundant by symmetr

They're just trying to illustrate 7 dimensional data. There are 67 2-dimensional comparisons, scatterplots, 67/2 if you eliminate ones made redundant by symmetry, 6*7/2+7 if you add 1-dimensional histograms for "self comparisons". So a scatterplot matrix, a splom, displays those 21 scatterplots in a meaningful way. Each "row" and "column" of scatterplots compares a single dimension against all of the others. You usually look at a thing like this to see if you'll get lucky and find a good "orthogonal" comparator—in other words, that just two normal, human-interpretable dimensions already form a good splitting plane like "elevation cross $/sq-ft" does in the running example.