Six colored squares, positioned at fixed coordinates and linked by a gray polyline, respond to mouse interactions in a unidirectional data flow. Clicking a square selects it, drawing a thick black stroke around it via `highlightSelectedDatum`. Dragging a square updates its x and y coordinates through d3’s `drag` handler, triggering a re-render that moves the square and the connecting line. The example uses d3 v7 with ES modules, rendering SVG elements through `join` and a centralized state managed by `setState`.