This example visualizes the top 15 most expensive cities by their mean Cost of Living Index using a bar chart. The code loads CSV data via `d3.csv`, filters by rank, and aggregates values with `d3.nest()` and `d3.mean`. It renders an SVG with `d3.scaleBand` for the x-axis and `d3.scaleLinear` for the y-axis, using `d3.axisBottom` and `d3.axisLeft`. Bars change color on mouseover and display a tooltip with city and index details.