Selecting a Year on a Line Chart
A line chart shows population over time for world regions, with a movable vertical line for selecting a specific year. Users drag the mouse across the chart, which updates the selected year and redraws the line. The visualization uses D3 v5 APIs including `scaleTime`, `scaleLinear`, `line`, `curveBasis`, `nest`, `axisLeft`, `axisBottom`, and `mouse`. Data comes from two CSV files (UN population estimates and medium variant) parsed with `timeParse`, and the chart renders as SVG with a color legend using `scaleOrdinal`.