it's not so much a d3 thing than it is an svg thing if i'm understanding it correctly. you just attach it as an attribute to a `line` it takes care of it. here's an [example definition](https://github.com/jdhenke/celestrium/blob/ed8f25da70daa429f44d378426b2d99c826ae2d4/core-coffee/GraphView.coffee#L47-L71) of the headers as well.