This visualization displays population trends over time for multiple cities, with each colored line representing a different region. A vertical black line marks the currently selected year, and moving the mouse across the chart updates this selection and re-renders the view. The code uses D3 v5 APIs including `nest()`, `scaleTime()`, `scaleOrdinal()`, `line()` with `curveBasis`, and axis generators. Data comes from the UN population estimates CSV files, processed by melting wide-format years into tidy rows, and rendered as SVG paths with a categorical color legend.