You are looking for a 2D library on top of SWT ??
The GEF project comes with a graphical library called Draw2d. This library can be used in any SWT application. Of course such applications are Eclipse’plug-ins but you can also use Draw2d in a standalone Java application with an SWT UI.
When dowloading the GEF plug-ins you get an eclipse plug-in called: org.eclipse.draw2d. This plug-in is in fact a “classical” jar without any Eclipse’s extension and can be used in any java application as a “classical” jar.
Here is a link to an Eclipse article introducing Draw2d.