So, to plot 3 items on a single graph we use ternary diagrams like the one you see above. At the three apexes, the composition would either be pure (100%) aromatics, pure naphthenes or pure paraffins (clockwise from top). Along any of the borderlines of this triangle, you're looking at a mixture of just 2 of these components (aromatics - naphthenes, or naphthenes - paraffins, or paraffins - aromatics). At any point within the triangle, the crude contains all three components, in varying degrees.
Take the example of 50 % aromatics to begin with. To plot this point on the graph, you'd create a drop a horizontal line about half way between the apex (100 %) and the base of the triangle opposite of that apex (0%), representing 50%. You repeat this process to locate the other %'s of the compound types on the graph, and the point you're after is the convergence of those three lines. Thus, the center of the triangle is: 33 %, 33 %, and 33 % of aromatics, naphthalenes and paraffin's, crude oil that would generally fall into the "old shallow" classification. Here is a dynamic exmaple.