This interactive line chart visualizes population trends over time by region, with a vertical black line indicating the currently selected year. Moving the mouse across the chart updates the selected year, repositioning the line and reflecting the change in the visualization. The chart uses d3.scaleTime and d3.scaleLinear to map data, d3.line with curveBasis for the paths, and d3.nest to group data by region. Data comes from UN population estimates and medium variant projections, rendered as an SVG using D3 v5.