A multi-line chart visualizes United Nations population estimates and projections from 1950 to 2100, grouped by world region with a color legend. A draggable vertical line marks a selected year, and moving the mouse over the chart updates that selection and re-renders the visualization. The code uses d3.csv, d3.nest, d3.scaleTime, d3.scaleLinear, d3.scaleOrdinal, d3.line with curveBasis, and d3.axisBottom/axisLeft, rendering SVG paths and circles.