Greetings,
It seems that package has been proposed couple of years ago. And
was rejected due to licence problems.
Here is the link :
http://sources.redhat.com/ml/cygwin-apps/2003-09/msg00348.html
However several things happend since then. First the AT&T labs at
cambrige is no more, and the graphviz package is now developed by
the graphviz.org. Consequently the licence has changed from some
weird proprietary AT&T code agreement, to standard CPL licence which
is an OSD approved licence.
http://www.opensource.org/licenses/cpl1.0.php
http://www.graphviz.org/License.php