A world choropleth uses color to encode each country's income economy, with a linked ordinal legend. Clicking a legend category filters the map's visual emphasis, dimming unselected countries and allowing a click again to clear the filter. Built with D3 v5, it combines `d3.geoPath` and `geoNaturalEarth1` projection for geographic shape rendering, `scaleOrdinal` and `schemeSpectral` for the color scale, and `d3.zoom` for pan and zoom. Data comes from the World Atlas TopoJSON and TSV datasets.