I try to use d3.scale.category.20b() to generate a color scale, problem is whatever number of the list I ask for, it always returns first element of the list.

```
var color = d3.scale.category20b();
console.log(color(X));

```

OR

```
console.log(d3.scale.category20b()(X);

```

No matter what X is, it always logs #393b79 which is the first elements, according to the [d3 API](https://github.com/mbostock/d3/wiki/Ordinal-Scales#wiki-category20c)