I had trouble getting ZGRViewer to open automatically from Drupal. I had to change the GraphViz executable paths in ZGRViewer directory preferences. I found where the executables were by typing in the terminal: which dot

For my install of graphviz-2.24.0.pkg the paths were:
GraphViz/dot executable =
/usr/local/bin/dot

GraphViz/neato executable =
/usr/local/bin/neato

GraphViz/circo executable =
/usr/local/bin/circo

GraphViz/twopi executable =
/usr/local/bin/twopi