Diagrams and Foreign Languages

For an image to be useful, the relevant or key results should be immediately obvious, so that the viewer need not spend a great deal of time first trying to understand what might be important. Diagrams can themselves be complex, requiring some time of study before they're understood, but the basic idea being expressed needs to be immediately apparent. It is similar to hearing someone speak in a foreign language, where even if it's not possible to understand the language (the specifics of the diagram) it's generally possible to tell whether they're angry or happy (what parts of the diagram are relevant and require further inspection).

— Ben Fry, Dissertation: Computational Information Design, 2004