This example shows a small directed graph with two nodes and one link, rendered as SVG shapes whose positions are driven by D3's force simulation. It demonstrates zooming and panning interactions, along with different node styles (gray and green circles) and a thick link line. The code is implemented using `d3.forceSimulation`, `forceManyBody`, `forceCenter`, `forceLink`, `drag`, `zoom`, and `symbol` from D3 v3, with data defined in `data.js` and the graph logic in `forceGraph.js`.