In the example, <http://mbostock.github.com/d3/ex/bubble.html>:

![Enter image description here](https://i.stack.imgur.com/WoLMG.jpg)

In line 27:

```
.style("fill", function(d) { return fill(d.packageName); });

```

Where is `fill()` defined? I didn't find it in `d3.js` either. And even if the package name is not a color, some random color is being assigned. How so?