For many educational applications such as learning tools for argumentation, structured diagrams are a suitable form of external representation. However, student-created graphs pose some problems to ITS designers, especially in defined domains. This paper demonstrates a graph-grammar-based approach for ITS construction in domains that benefit from diagram representations. For these, graph grammars offer some helpful affordances. They make it easy to express possible manipulations by which a student might create a diagram, they facilitate the definition of structurally complex and pedagogically interesting constellations of graph elements to which an ITS should respond with feedback messages, and they offer a general parsing mechanism that allows the analysis of student-created diagrams by recognizing these constellations in graphs.