This example demonstrates a unidirectional data flow pattern: a circle follows the mouse cursor while loading and displaying the Iris dataset. The visualization first fetches `data.csv` containing sepal and petal measurements, parses it with `csvParse`, and converts the numeric string values to numbers. A `pre` element displays the JSON-formatted state data, which transitions from a string `"loading"` to the complete dataset. The code uses D3 v7's `select` and `join` APIs to manage the DOM and implements explicit `setState`/`render` functions to maintain predictable one-way data flow.