This stacked bar chart visualizes the estimated religious composition of the 20 most populous countries in 2010, using data from the Pew Research Center's Global Religious Landscape. The chart uses `d3.flatRollup` and `d3.stack` to structure the data, with `d3.scaleLinear` for the x-axis (population in millions) and `d3.scaleBand` for the y-axis (countries). A color legend, built with `colorLegend.js`, allows users to hover over a religion to highlight its segments across all bars while fading others, using `d3.scaleOrdinal` with `schemeCategory10` for color assignment.