This example demonstrates interactive selection of circles rendered with D3.js. Each circle displays its own position and radius, and clicking one highlights it with a black stroke and raises it above the others. The visualization uses a unidirectional data flow pattern where state changes trigger re-rendering through the `viz` function. D3’s `selectAll` and `join` methods manage circle creation and updates, while click event listeners call `setSelectedDatum` to update the selected circle’s styling.