This example demonstrates interactive brushing on a scatterplot of six circles, where dragging a rectangular selection highlights overlapping data points. It shows how a brush can filter or emphasize specific marks in a visualization. The implementation uses D3’s `brush` module, with a `brush` event listener that checks each circle’s coordinates against the selection bounds to update a `highlightedSet` in the state. Circle styling (stroke, opacity) responds to the highlighted set, and the `end` event clears highlights when the brush is dismissed. The rendering uses SVG elements via D3’s `select` and `join` pattern.