This chart shows regional population trends from 1950 to 2100, drawing on United Nations population estimates and medium-variant projections. A vertical line marks the year 2020, and a color legend identifies each region. The code uses d3.nest and d3.scaleOrdinal to group and color the data, d3.line with curveBasis for smooth paths, and SVG for rendering. Separate modules handle data melting, the line chart, and the color legend.