This visualization shows population trends over time for world regions from 1950 to 2100, displayed as multiple smoothed line paths colored by region. The code uses D3 v5 with `d3.nest` to group data by region, `d3.line` with `curveBasis` for smoothing, and `d3.scaleTime`, `d3.scaleLinear`, and `d3.scaleOrdinal` for axes and color mapping. Data is melted from wide UN population CSV files using a custom `melt` function that reshapes yearly columns into long format, combining estimates and medium-variant projections.