This example shows a multi-line chart tracking population over time for different world regions, with a vertical black line marking a selected year. Moving the mouse across the chart updates the selected year by inverting the x-scale position, and a color legend identifies each region. The visualization uses d3.line with curveBasis for the smoothed paths, d3.scaleTime and d3.scaleLinear for axes, and d3.nest to group the data by region. Population data comes from UN estimates and medium variant projections, rendered as SVG.