This example demonstrates a simple interactive visualization where a circle follows the mouse cursor across the screen, using a unidirectional data flow pattern. The code uses d3-selection’s `select` and `selectAll` with data joins to create and update an SVG element. Mouse movement events trigger a `setState` function that updates an immutable state object with the cursor’s `x` and `y` coordinates, which then re-renders the circle’s `cx` and `cy` attributes. The rendered circle has a fixed radius of 50 pixels, and the SVG fills the entire viewport.