Where is this fill function defined?
<p>In the example here - <a href="http://mbostock.github.com/d3/ex/bubble.html" rel="nofollow">http://mbostock.github.com/d3/ex/bubble.html</a></p>

<p>In line 27:</p>

<pre><code>.style("fill", function(d) { return fill(d.packageName); });
</code></pre>

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

<p>Sorry if its a trivial question, javascript newbie here.</p>
