takes geojson or topojson and uses d3 (possibly with the help of topojson) to add it to a leaflet map as a layer. Bassed off this example

not sure if it actually solves any problem besides helping me learn d3.

api:

L.d3(data,options);
//or
new L.D3(data,options);

where data is either a url to geojson or topojson data or it can be an object, and options include:

• "topojson" which is false if not topojson or the name of the topojson object you want.
• "pathClass" the class your paths should have, this defaults to "path" and can be a string or a function which recieves the feature and returns the path string.
• "type" whether d3 should download and treat it as json, csv, xml, text, tsv, html. Considering how this library makes some pretty strong assumes re it being json I'd not touch this. Forget I put this here.

Your going to want to add css styles manually, note that by default csv closes paths and fills them for you if you don't specify "fill:none;" Also I'd avoid styling "path" as that will effect all leaflet paths, including regular ones made inside leaflet.

the layer also has a method "bindPopup" with one argument which is content. This binds a popup to the layer with the content set to content, which may be a string which is just passed on unchanged or a function which is called on the properties of the feature clicked question.

added two more options, svgClass which just takes a string which lets you set a class on the whole layer, useful for things like colorbrewer which expect this, and "before" which takes a function and is called with the data as an argument and the feature class as this. useful if you need to calculate stats for a range. its called right before the feautes are added to the map and right after path and bounds are calculated.