The concept of the diagram appears in A Thousand Plateaus (ATP 141-144, 531 n. 41 / 176-180, 177 n. 38) but the details of its development are found in Guattari's writings of the 1970s. The notion was adapted from Charles Sanders Peirce, who includes the diagram among the icons in his index-icon-symbol model of the sign. Peirce identifies three types of icon: image, metaphor, and diagram. For him, the icon operates through a relation of resemblance between the sign and its referent. Guattari would agree that the image and the metaphor signify through resemblance, which is to say representation, but his version of the diagram functions differently because as he defines it, the diagram does not signify; it is "a-signifying"... Examples of the diagram at work include the algorithms of logic, algebra and topology; as well processes of recording, data storage, and computer processing; all of which are used in mathematics, science, technology and polyphonic music. Neither mathematics nor musical notation are languages -- rather, both bypass signification altogether...

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In other words, diagrams do not represent thought; rather, they generate thought.