A graph (as in "graph theory") is a discrete mathematical structure which deals with objects and pairwise relations between them. Graphs have immense applications in various disciplines of science. Knowledge of this subject is essential for mathematics and computer science enthusiasts.
D3 Graph Theory is project aimed at anyone who wants to learn graph theory. It provides quick and interactive introduction to the subject. The visuals used in the project makes it an effective learning tool.
And yes, it is open-source. Check the code at GitHub. Created by Avinash Pandey, a mathematics and computer science enthusiast.