Looks fine but I would format this to stay consistent with the rest, one liners are nice but not always easy to read.

```javascript
on("mousemove", function() {
  return tooltip.
    style("top", (d3.event.pageY - 10) + "px").
    style("left", (d3.event.pageX + 10) + "px");
}).
```

Once you do that I'll merge.