Gaphas is a user interface component (widget) for working with diagrams.
Diagrams can be crafted and exported to a variety of formats, including SVG and PDF.
The modular design makes it easy to extend and reuse this component for various tasks, including modelling (e.g. Gaphor), EDA and system modelling.
This document describes some of the basic ideas of Gaphas.
Also, take a look at the Gaphas tech site on http://gaphor.devjavu.com/wiki/Tech.