This visualization shows the average sepal length for each Iris species in the classic Fisher dataset. The example fetches a CSV file and parses it with `d3.csvParse`, converting numeric fields, then uses `d3.flatRollup` with `d3.sum` and array length to compute the per-species mean. The result is displayed as a formatted JSON string inside a `<pre>` element, which re-renders whenever the data changes.