Cannot install graphviz with macports
Sometimes when you install command line software, the software is actually a package that contains a collection of binaries (or library source files) and according to the Portfile description of graphviz, it's one of those packages:


  The Graphviz layout programs dot, neato, fdp, sfdp, twopi and circo
  take descriptions of graphs in a simple text language, and produce
  output in a variety of visual and text formats.


I personally don't use MacPorts but according to this answer it seems like you can view the binaries (and library source files) installed by a package through:

port contents installed_port_name