Seven semitransparent circles sit at fixed positions on a light-green SVG canvas. Clicking any circle selects it, giving it a thick black stroke and bringing it to the front. The visualization is built with D3 v7 using a unidirectional data flow pattern where the viz function re-renders on each state change. The code uses d3.select and join to manage circle elements, while event listeners call a setter to update the selected datum.