This example demonstrates a paint tool where a circle follows the mouse and leaves a trail of colored dots. Users draw by moving the pointer, cycle through nine colors by clicking, and adjust brush size from five preset values by right-clicking. The visualization uses a unidirectional data flow with a custom `viz` function and a `setState` callback. D3 selections handle mouse events and append SVG circles dynamically, while a text label shows the "Paint!" instruction in the center-top of the viewport.