This example demonstrates interactive brushing on a set of five colored circles. Users can drag a rectangular selection region, and any circle whose center falls within that area is highlighted with a thick black stroke. The visualization updates its state through D3's `brush` API, using the `on('brush')` and `on('end')` events to track the selection. The circles are rendered as SVG elements with hardcoded coordinates and colors, and the highlighted set is managed via a `Set` stored in the component’s state.