This example shows a choropleth of U.S. counties where each county’s color encodes its geographic area on a logarithmic scale from brown (small) to steelblue (large). The map renders county boundaries as SVG paths using `d3.geo.path()` and TopoJSON, with state outlines drawn as white strokes. The fill color is computed by passing each path’s area, obtained via `path.area(d)`, into a `d3.scale.log()` with a domain of 10 to 500. The data comes from a local `us.json` TopoJSON file, and the visualization uses d3.v3.