What is a logic diagram?

Robert Brandom's expressivism argues that not all semantic content maybe made fully explicit. This view connects in interesting ways with recentmovements in philosophy of mathematics and logic (e.g. Brown, Shin, Giaquinto) totake diagrams seriously−as more than a mere 'heuristic aid' to proof, but eitherproofs themselves, or irreducible components of such. However what exactly is adiagram in logic? Does this constitute a cleanly definable semiotic kind? The paperwill argue that such a kind does exist in Charles Peirce's conception of iconic signs,but that fully understood, logical diagrams involve a structured array of normativereasoning practices, as well as just a 'picture on a page'.KeyWords.logic, mathematics, diagram, proof, icon, existential graphs,expressivism, pragmatism, Peirce, Brandom, Ayer.