This interactive world map visualizes country populations with a sequential YlGnBu color scale. It loads TopoJSON world boundaries via `d3.json` and population data from a CSV, then joins them through ISO country codes. The SVG map uses `d3.geoNaturalEarth1` and `d3.geoPath` to render countries, with hover tooltips and an axis-labeled gradient legend built using `d3.scaleSequential` and `d3.axisBottom`. Missing values are shown in gray.