This example demonstrates how to fetch and transform CSV data from a remote URL using D3 v7. It loads the classic Iris flower dataset, parses it with `csvParse`, converts string values to numbers, and then applies `flatRollup` to calculate the average sepal length for each species. The resulting summary is rendered as a JSON object in a `<pre>` element, showing a simple text-based visualization of aggregated data. The code uses `fetch`, `select`, and `setState` for asynchronous data loading and re-rendering.