min axis getter

According to documentation (http://c3js.org/reference.html#api-axis-min) with

``` javascript
chart.axis.min()
```

I should get a dictionary containing min values of each chart axis but they appear as Undefined

```
Object { x=undefined, y=undefined, y2=undefined}
```

Is this correct?

Start with the documented chart.axis.min() entry point and reproduce the reported result using a minimal chart. Compare the returned x, y, and y2 values with the API reference; done when the implementation and documentation agree about whether the getter returns each axis's minimum.

Written by the indexing model from the issue text.