This article suggests a tool for analysing diagrams in mathematical texts. The starting point for this tool is Halliday's Systemic Functional Grammar (SFG), which considered language as a social practice. Kress and Van Leeuwen (2006) applied Halliday's SFG to images, suggesting a grammar for reading images. Morgan (1996) adopted Halliday's SFG and developed a linguistic tool to analyse mathematical texts. This article extends their work, classifying geometric diagrams as either narrative or conceptual, depending on whether the diagram contains traces of temporality. The article concludes by applying the analytic tool to two mathematical texts and discussing possible uses and implications.