Two different concepts of iconicity compete in Peirce's diagrammatical logic. One is articulated in his general reflections on the role of diagrams in thought, in what could be termed his diagrammatology – the other is articulated in his construction of Existential Graphs as an iconic system for logic representation. One is operational and defines iconicity in terms of which information may be derived from a given diagram or diagram system – the other has stronger demands on iconicity, adding to the operational criterion a demand for as high a degree of similarity as possible.This paper investigates the two iconicity notions and addresses some of the issues they involve.