A choropleth map displays world countries colored by income group, with a color legend for filtering. Clicking a country highlights it and dims all others; clicking again resets. The map uses D3's `geoNaturalEarth1` projection with `geoPath` for rendering shapes, and supports zooming via the `zoom` event. Data is loaded from a CSV of travelers and a TopoJSON world atlas, processed with `topojson.feature`. An ordinal spectral color scale maps economy categories.