This visualization presents population trends over time for world regions using a multi-line chart, with a vertical line marking the currently selected year. Users move the mouse across the chart to change the selected year, which updates the line position. The chart is rendered as SVG using D3 v5, employing scales like `scaleTime` and `scaleLinear`, the `line` generator with `curveBasis`, and nested data via `nest`. A separate color legend maps each region’s line to a categorical color scale.