This bar chart shows the number of runners in each age group at the 2024 Boston Marathon, with bar colors encoding the average percent change between each group’s first and second half finish times. Using React and D3 v7, the chart groups runners from a CSV file into age categories, counts them, and calculates an average split-time change. The `viz` function builds the chart with D3’s `scaleBand` and `scaleLinear`, and a custom `getColor` utility maps average percent-change values to a green-to-red palette. A legend explains the color scale.