From the webpage:
The Diagram Editor Generator
DiaGen
DiaGen is a system for easy developing of powerful diagram editors. It consists of two main parts:
- A framework of Java classes that provide generic functionality for editing and analyzing diagrams.
- A GUI tool (the DiaGen designer) for specifying the diagram language and automatically generating a visual editor from this specification.
The combination of the following main features distinguishes DiaGen from other existing diagram editing/analysis systems:
- DiaGen editors include an analysis module to recognize the structure and syntactic correctness of diagrams on-line during the editing process. The structural analysis is based on hypergraph transformations and grammars, which provide a flexible syntactic model and allow for efficient parsing.
- DiaGen has been specially designed for fault-tolerant parsing and handling of diagrams that are only partially correct.
- DiaGen uses the structural analysis results to provide syntactic highlighting and an interactive automatic layout facility. The layout mechanism is based on flexible geometric constraints and relies on an external constraint-solving engine.
- DiaGen combines free-hand editing in the manner of a drawing program with syntax-directed editing for major structural modifications of the diagram. The language implementor can therefore easily supply powerful syntax-oriented operations to support frequent editing tasks, but she does not have to worry about explicitly considering every editing requirement that may arise.
- DiaGen is entirely written in Java and is based on Java SE (Version 6 is required). It is therefore platform-independent and can take full advantage of all the features of the Java2D graphics API: For example, DiaGen supports unrestricted zooming, and rendering quality is adjusted automatically during user interactions.
DiaGen uses hypergraph grammars to specify diagram languages. While this approach is powerful and its theory is very clear, it is hard for the unexperienced user to model an editor. A very common solution to this problem is meta-modeling, which is used by DiaMeta.
DiaMeta
DiaMeta allows to use meta models instead of grammars to specify visual languages. The current implementation employs the Eclipse Modeling Framework (EMF). Additionally, support for MOF (via MOFLON) is currently added. Editors generated by DiaMeta have the same benefits as those generated by DiaGen, and they show a similar behaviour like DiaGen.
If you need a custom diagram editor, this may be a good place to look around.