This example presents an interactive choropleth map of census tracts in the Rockville, Maryland area, colored by population density using a `d3.scaleThreshold` and the `d3.schemeOrRd` color ramp. A draggable yellow circle, implemented with `d3.drag`, allows the user to move it across the map; as it moves, a tooltip displays the name of the tract currently under the cursor, determined via `d3.geoContains`. The map is rendered as SVG paths projected with `d3.geoConicConformal` and `d3.geoPath`, using GeoJSON converted from TopoJSON, with population data joined from a separate JSON file.