Graph Editor - design - 3 layers

1. model for nodes / graphs - contains no display information - this is what
would be browsed / manipulated by the computer
	- node
		- uid (use ruby id for the moment, don't know about
		  persistence)
		- child nodes, indexed in some way
		- parent nodes, and indexes of this node in each
		- ? data, or url to data
		- ? type
		- ? name (maybe)
		- facility to create layout peers

2. layout - this is specifying shapes, colours, labels, etc. there may be more than one layout (diagram) for a particular node - the layout must reflect the underlying model, and should be able to send signals back to change the model when necessary?
	- node-layout
		- world coordinates
		- shapes
		- colours
		- labels
		- facility to create view peers (i.e. canvas items / canvas
		  group), given a view (i.e. canvas)
	- different

3. views - tk canvases and canvas-items or other such visible images - there may be more than one view on a particular diagram, with different viewpoint (pan, zoom) - these are also responsible for getting user input, and sending signals back to the layout
	- canvas
	- canvas-item
	- canvas-group