A pie chart visualizes the species distribution of the Iris dataset, showing each category as a slice with its own color and a corresponding legend. The code loads CSV data from a GitHub gist, groups it by species to count occurrences, and uses `d3.pie` and `d3.arc` to generate the SVG paths. Colors are assigned via `d3.scaleOrdinal` with a manual range, and `d3.select` manages the SVG elements while `d3.rollups` performs the aggregation.