This interactive line chart visualizes population changes over time for world regions, with a vertical line marking a user-selected year. Clicking or hovering along the chart updates the selected year, repositioning the line to show the population at that moment. The visualization uses d3.nest and d3.scaleOrdinal to group data by region, d3.line with curveBasis to draw smoothed paths, and d3.axisBottom and d3.axisLeft for labeled axes. Data comes from UN population estimates and projections split across two CSV files, loaded via d3.csv.