Fix Graphviz bug when graphviz not installed natively

[mosser]

When Graphviz is not installed locally (and available in $PATH), the graph library used to draw the diagram default to a javascript implementation that does not compute text layout properly. We need to put as a prereq that graphviz needs to be installed, and force the code to use this rednering mode instead of defaulting to Javascript. That might also be linked to some problems @aaronloh16 was having related to the "rough" mode.