Select a vertical area on a chart

I created the following feature using C3 & jQuery:
http://jsfiddle.net/prqwrjtq/2/

You can select an area on the chart. I would like to implement this ability as C3 API. It would accept a callback with start and end points as arguments, so a user can select an area on the chart between two data points and get a response to their selection.

e.g
```
c3.generate({
...
areaselect: {
enabled: true,
onselect: function(a, b) {
// do something with data points here
},
color: #999999 // selector color
}
})
```

How would one go about this?