This example shows a directed graph of university courses, with nodes representing courses and links indicating prerequisites. Users can drag nodes to reposition them; double-clicking a node releases it from its fixed position, while dragging fixes it in place. The visualization uses D3 v3’s `d3.layout.force` for physics-based layout, with `d3.json` loading the course data from `graph.json`. Node and link positions are updated during the force simulation’s `tick` event, although the tick handler itself is left unimplemented, so nodes remain static initially. Rendering is done via SVG elements styled with CSS.