Melting Data for Multiple Lines
This example visualizes population over time by region using multiple smoothed lines, with a color legend identifying each region. The data is loaded from UN population estimate CSVs, then transformed using a custom `melt` function that reshapes wide-format data into long format for D3 line generation. The visualization uses `d3.scaleTime`, `d3.scaleLinear`, `d3.nest`, and precomputed curve paths with `d3.line` and `curveBasis`. Axes are rendered with `d3.axisBottom` and `d3.axisLeft`, while a custom legend component manages color-coded circles using `d3.scaleOrdinal` and `schemeCategory10`.