Diagrams are based on origins of :
Resemblance: The most naive account (more likely to be made of diagrams with a strongly pictorial element) is simply that they resemble the things they refer to.
Metaphor: It is possible to depict things that do not exist physically, that we, humans, have a mental image of them (e.g. abstract concepts, or structure representations in computer systems)
Concrete Descriptions: By abstracting physical entities we can get a better picture of the world around us.
thanassis.com