This paper responds to renewed interest in the centuries old question of what is a diagram. Existing status of our understanding of diagrams is seen as unsatisfactory and confusing particularly when responding to problems of accounting for the variety of diagrams, handling change in diagrams in a well formed way, and all of this in the context of semantically mixed diagrams. Existing frameworks (Peirce, Bertin, Engelhardt) for understanding diagrams are discussed and evaluated against these problems. These frameworks are then contrasted for similarities and differences with our proposal which is based on, SySpM, Symbolic and Spatial Mapping. Potential and contributions of the proposed framework are briefly mentioned.