This example shows a line chart of population over time by region, with a vertical line marking a selected year that updates as the mouse moves across the chart. The chart uses d3.line with curveBasis to draw the paths, d3.nest to group data by region, and a mouse-interceptor rectangle to detect hover position, calling setSelectedYear via d3.mouse. Data comes from UN population estimates CSV files, merged from two sources for years 1950–2100, and is rendered with SVG. A color legend is included using a custom colorLegend module.