This example visualizes population over time by region, showing multiple lines that trace changes from 1950 to 2100. It uses D3 v5 with SVG rendering, employing `scaleTime`, `scaleLinear`, and `scaleOrdinal` for scales. The code melts wide-format UN population data into tidy rows using a custom `melt` function, then nests it with `d3.nest()` to generate paths via `d3.line()`. A color legend and gridlines are added, with data sourced from UN population estimates and medium-variant projections.