Information graphics or diagrams are two-dimensional maps of relationships that present information. Creators and interpreters of diagrams need to know what makes some diagrams more effective in communicating information. This paper suggests a theoretical framework for diagram classification to make this possible. This framework consists of two related continua along the x-axis: categories of information structure, ranging from equidistant interval through ordinal to nominal, appear in the upper, horizontal coordinate; and categories of layout and design, ranging from technical through formal to informal, appear along the lower, horizontal coordinate. The definition of equidistant interval/technical arrays provides a clear division from the rest of the horizontal continuum. The definitions of ordinal and nominal categories also yield an explicit break. The formal/informal boundary is not as clearly defined. As a result, the farther a diagram moves to the right along the framework, the more intuitive the decisions become regarding diagram features. From this framework, research questions into graphic literacy could begin by focusing upon the effectiveness of various diagrams based on the type of communication used to convey information. (Contains 14 references.) (JLB)