drag = {
function dragstarted() {
d3.select(this).attr("stroke", "black");
}
function dragged(event, d) {
d3.select(this).raise().attr("cx", event.x).attr("cy", event.y);
}
function dragended() {
d3.select(this).attr("stroke", null);
}
return d3.drag()
.on("start", dragstarted)
.on("drag", dragged)
.on("end", dragended);
}