Comment 2 for bug 163142

libgraphviz3 is the Ubuntu package which contains the library bits of graphviz. At the moment it depends on freetype, glib, cairo, some X libs and a few others; built with GTK support, it pulls in libatk and libgtk (and maybe some others), which not everyone wants to install. At least it's not libgnome and friends...

The bug is really that graphviz doesn't build-dep on gtk. Whether a separate package is required for the gtk plugin depends on whether it is desirable to preserve the existing separation between graphviz and graphviz-cairo. I'm not personally bothered either way :-)