A diagram is a change of a state represented in a coordinate system. The coordinates of a diagram can be value-related and time-related or rubric-related. Physical quantities or monetary amounts can be entered on the value coordinate, relative or absolute time values or rubrics can be entered on the time or rubric-related coordinate.
The coordinate system chosen for the representation can be two- or three-dimensional, it can be rectangular and have linear coordinate values, but it can also be circular. The coordinates start at the reference pointzero, the division of the coordinates can be linear, logarithmic, quadratic or exponential.
The representation of the diagram within the coordinate system can be line-shaped as line diagram, point-wise as point diagram, by individual bars as bar diagram or as circle diagram. Furthermore, there is the Entity Relationship Diagram( ERD), a representation form in which entities are set in relation to each other. Other diagrams are the block diagram in which functional units are connected by lines and the flowchart in which the logical steps of a problem or program flow are represented with special symbols.
Diagrams should visually show the viewer trends and state changes without having to interpret long columns of numbers. It is clear and should be structured in such a way that changes and developments can be grasped directly. A chart relates two or three different variables to each other. For example, the turnover of a company over time, or the sound pressure inrelation to the distance, or the characteristic impedance to the frequency, or the field strength at a distance from the antenna.