This example visualizes average weight differences before and after treatment, grouped by gender, using diverging blue and pink bars. It builds a React component that loads CSV data via `d3.csv`, nests and aggregates it with `d3.nest` and `d3.mean`, then renders an SVG bar chart with `d3.scaleBand` and `d3.scaleLinear` scales. Axes are drawn using `d3.axisBottom` and `d3.axisLeft` via refs, while the bar heights encode positive and negative weight changes relative to a zero baseline.