Diagrams are used heavily throughout contemporary society, yet there is little guidance for educators on teaching students to think critically about diagrams in general. Because of this there is almost no systematic guidance for learners on how to approach unfamiliar diagrams with some degree of confidence, particularly in terms of their ability to think critically about the visual ‘rhetoric’ of a given diagram. This is a crucial ability to foster in students, given society’s increasing reliance on forms of graphical communication which encourage the use of diagrams (internet, PowerPoint, tablet computing, smart phones) and the decision-making contexts in which diagrams are used (climate science, stock prices, corporate strategy). While we teach students to read information from specific forms of diagrams within specific contexts, we rarely teach students to think critically about, for example, the manner in which a diagram presents data in such a way that the presentation generates a signification for the data which is not necessarily inherent in the data itself, but is definitely inherent in the manner it is visually presented.