A multi-line chart displays population trends over time for various countries and regions, using the United Nations population estimates and projections dataset. The visualization employs multiple vertical reference lines to highlight the current year, a hovered year, and a selected year, which users can change by clicking on the chart. The D3 code uses `d3.nest` to group data by region and `d3.line` with `d3.curveBasis` to draw the smoothed paths, while `d3.scaleOrdinal` and `d3.schemeCategory10` assign colors. A custom color legend function renders the region names and color swatches.