The dot layout engine assumes a spline smoothing algorithm for edges, rather than Tk's default smoothing. So edges rendered to Tk canvas will look better if the Tkspline extension is available. Tkspline is available from: ftp://ftp.graphviz.org/pub/ For Tk prior to tk8.3.0 Jan Nijtmans' dash patches are required by Tkspline. Dash patches are available from: http://home.wxs.nl/~nijtmans/