Diagrams are widely used by people across many disciplines and applications. Often such diagrams express a range of different semantics thus making it difficult not only for people who interpret them but also for those who develop software to draw them. The nature and underlying issues of this problem are explored. A meta framework for a good understanding of diagrams based on Symbolic and Spatial Mappings is proposed and briefly outlined. The potential and richness of the framework is demonstrated by applying it to interpret an Ordnance Survey map. A brief discussion of the framework's potential for guiding the development of diagramming software is presented