Scientific diagrams could and did appear in unexpected places. This essay discusses such an example: the diagrams that the thirteenth- to fourteenth-century scholar George-Gregory Chioniades added to a manuscript of John of Damascus’s Fountain of Knowledge as part of his commentary on the text. I argue that the diagrams were a very important, if not the most important component of his commentary. They allowed Chioniades to (1) shape how readers would experience and study John of Damascus’s philosophical exposition and (2) update the Damascene’s introductory discussions on astronomy and astrology with Arabic terminology and his own interests in medical astrology. By paying attention to such diagrams, historians have much to learn about the intellectual projects of scholars like Chioniades. Conversely, Chioniades’ own testimony about his diagrams can help us better understand how medieval scholars thought about, and thought with, the diagrams they have left behind.