A diagram is a symbolic representation of information using visualization techniques. Diagrams have been used since prehistoric times on walls of caves, but became more prevalent during the Enlightenment.[1] Sometimes, the technique uses a three-dimensional visualization which is then projected onto a two-dimensional surface. The word graph is sometimes used as a synonym for diagram.
In science the term is used in both ways. For example, Anderson (1997) stated more generally: "diagrams are pictorial, yet abstract, representations of information, and maps, line graphs, bar charts, engineering blueprints, and architects' sketches are all examples of diagrams, whereas photographs and video are not".[2] On the other hand, Lowe (1993) defined diagrams as specifically "abstract graphic portrayals of the subject matter they represent".[3]
In the specific sense diagrams and charts contrast with computer graphics, technical illustrations, infographics, maps, and technical drawings, by showing "abstract rather than literal representations of information".[4] The essence of a diagram can be seen as:[4]
Or in Hall's (1996) words "diagrams are simplified figures, caricatures in a way, intended to convey essential meaning".[5] These simplified figures are often based on a set of rules. The basic shape according to White (1984) can be characterized in terms of "elegance, clarity, ease, pattern, simplicity, and validity".[4] Elegance is basically determined by whether or not the diagram is "the simplest and most fitting solution to a problem".[6]
Many of these types of diagrams are commonly generated using diagramming software such as Visio and Gliffy.
Diagrams may also be classified according to use or purpose, for example, explanatory and/or how to diagrams.