The intelligent diagram is a recent metaphor for diagramming in which the underlying graphic editor parses the diagram as it is being constructed, performing error correction and collecting geometric constraints that capture the relationships between diagram components. During diagram manipulation a constraint solver uses these geometric constraints to maintain the diagram's semantics. We introduce the Penguins system. This automates the development of graphical editors that support the intelligent diagram metaphor. It takes a grammatical specification of a particular diagram language and generates an editor specialized for the creation, manipulation and parsing of diagrams in that visual language. We extend previous research in this area by allowing more expressive grammars, performing automatic error correction, and detailing how efficient incremental parsing has been achieved. We also provide an empirical evaluation of the system. This shows that the system can be used to generate customized editors for a wide variety of diagram languages, ranging from state transition diagrams to mathematical equations, with real-time incremental parsing and error correction.