This interactive stacked bar chart displays the top 20 countries by population, broken down by religious affiliation in millions. Users can hover over the color legend to highlight the corresponding segment across all bars while fading others. Built with D3 v7, it uses `flatRollup`, `stack`, and `scaleBand` for data aggregation and layout, plus custom mouse events for filtering. The chart draws from a CSV of religion-by-country data and includes responsive axes and a categorical color scale.