  As the use and diversity of diagrams across many disciplines grows, there is
an increasing interest in the diagrams research community concerning how such
diversity might be documented and explained. In this article, we argue that one
way of achieving increased reliability, coverage, and utility for a general
classification of diagrams is to draw on recently developed semiotic principles
developed within the field of multimodality. To this end, we sketch out the
internal details of what may tentatively be termed the diagrammatic semiotic
mode. This provides a natural account of how diagrammatic representations may
integrate natural language, various forms of graphics, diagrammatic elements
such as arrows, lines and other expressive resources into coherent
organisations, while still respecting the crucial diagrammatic contributions of
visual organisation. We illustrate the proposed approach using two recent
diagram corpora and show how a multimodal approach supports the empirical
analysis of diagrammatic representations, especially in identifying
diagrammatic constituents and describing their interrelations in a manner that
may be generalised across diagram types and be used to characterise distinct
kinds of functionality.
