This example visualizes population trends for countries over time as multiple smoothed lines, using a nested data structure to group values by region. The chart displays yearly population estimates from the UN’s medium variant dataset, covering 1950 to 2100. Built with React and D3 v5, the code employs `d3.nest` for data grouping, `d3.line` with `curveBasis` for path generation, and `d3.scaleOrdinal` with `schemeCategory10` for color mapping. A custom color legend and axes with tick formatting complete the SVG rendering.