Other useful data types are geospatial coordinates, often used for weather or population data; anything where you want to draw a map.
d3.js gives us three tools for geographic data: paths produce the final pixels, projections turn sphere coordinates into Cartesian coordinates, and streams speed things up.
The main data format we'll use is TopoJSON, a more compact extension of GeoJSON, created by Mike Bostock. In a way, TopoJSON is to GeoJSON what DivX is to video. While GeoJSON uses the JSON format to encode geographical data with points, lines, and polygons, TopoJSON instead encodes basic features with arcs and re-uses them to build more and more complex features. As a result, files can be as much as 80 percent smaller than when we use GeoJSON.