This bar chart displays the top 15 most expensive cities by their mean cost of living index, sourced from a CSV file. Using D3 v4, the code filters data by rank, nests entries by city, and computes averages with `d3.nest().rollup()`. It renders an SVG with `d3.scaleBand` and `d3.scaleLinear` axes, while interactive tooltips appear on mouseover via `d3.select` and style changes.