Subchart selector does not work with selection < tick

* **C3 version**: 0.4.22 (at least)
* **D3 version**: 3.5.6
* **Browser**: Firefox 59
* **OS**: Linux

When we select a period between 2 ticks in the subchart, nothing appears on the main chart.


See a more simple example https://codepen.io/anon/pen/aYyVLv with code below:

```js

var chart = c3.generate({
data: {
x: 'x',

type: 'bar',

columns: [
['x', '2013-01-01', '2013-01-02', '2013-01-03', '2013-01-04', '2013-01-05', '2013-01-06'],
['data1', 30, 200, 100, 400, 150, 250],
['data2', 130, 340, 200, 500, 250, 350]
]
},
axis: {
x: {
type: 'timeseries',
tick: {
format: '%Y-%m-%d'
}
}
},
subchart: {
show: true
}
});
```