This interactive choropleth map shows cumulative COVID-19 cases per U.S. state as of September 2021, with darker fills representing higher case counts. Clicking a state displays a tooltip with its name, total cases, and total deaths. The visualization loads county-level data from a CSV via `d3.csv`, aggregates it by state using `d3.rollup`, and renders SVG paths from TopoJSON using `d3.geoPath`. Color opacity is scaled proportionally to each state’s case total, with a bivariate legend circle appended on click.