Graph Theory is pretty widely applicable. It's been a core course in my undergraduate program for years (I think since the beginning of the program). For exampl

Graph Theory is pretty widely applicable. It's been a core course in my undergraduate program for years (I think since the beginning of the program). For example, you can apply it to a number of different domains in physics. It can be used to analyze electrical circuits, pipe flow, and mechanics. It's at the core of a number of packages related to these topics. The circuit simulator SPICE uses graph theory as does Modelica and MapleSim. That's only the physics applications. I've read papers that use graph theory in a lot of domains. There was a good book I used as reference when I looked at using graph theory for CAD (as part of an undergrad project), but I don't recall it as that was years ago. I very likely would have done a graduate degree under a professor studying graph theory (he was my 4th year project supervisor), but he died over New Year's that year and I had to change supervisors for my last four months. But, he was the reason I decided to go to grad school in the first place.