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Zgraph is publically available software which runs on Symbolics Lisp Machines, release 6 and TI 
Explorer release 3.0.  Conditional compiles handle both cases.  Please feel free to 
distribute it (free of charge) to others.  I would appreciate receiving any improvements you
make which you would like me to distributed to others.  In particular, I've left hooks for the
user to plug in his or her own vertex placement algorithm and would be interested in forming any
new ones.

Zgraph has the following problems:
1. The vertex placement algorithm suffers from performance problems on huge 
graphs and doesn't necessarily produce elegant results.  
2.  Trees and lattices are currently not treated specially, so their layout tends to look
unnatural.
3.  The graph output code needs to be speed up so that huge graphs don't take forever
to output.  Vertex clipping has been implemented so that panning doesn't cause the entire
graph to be redrawn when only a portion of it is visible; however, the inverse hack of
avoiding the drawing of too much detail when huge graphs are zoomed outwards has not been
implemented.  This deficiency causes huge graphs to appear junky (until they are zoomed 
inwards), as well as being slow to draw.  

On the other hand, the user interface is in good shape and there's plenty of on-line
documentation.  I hope you will find it a useful debugging tool.

--John Hogge