When editing a graph dot file in an external editor then saving the changes, Graphviz.app doesn't alway (almost never) re-reads the changed file and update the graph accordingly. Also when adding a new edge the graph typically completely disappears and leaves a dark gray screen that has to be closed, and the graph file re-opened. Sometimes when adding an edge it doesn't do this though. When syntax errors are present in the graph file, it won't re-read the contents after they have been corrected. I'm running this on the latest version of Mac OS X Yosemite (10.10) on the newest 15" Macbook Pro with retina display (mid 2014 model)