Timeseries chart tooltips break with different length datasets

Tooltips that you see on mouseover break for timeseries chart if short datasets are loaded first.

Working example - longest dataset first.

```
var chart = c3.generate({
data: {
x: 'x',
json: {
'x': ['2013-01-01', '2013-01-02', '2013-01-03'],
'data1': [10, 20, 30],
'data2': [10, 20],
'data3': [10]
}
},
axis: {
x: {
type: 'timeseries',
tick: {
format: '%Y-%m-%d'
}
}
}
});
```

Broken example - shortest dataset first:

```
var chart = c3.generate({
data: {
x: 'x',
json: {
'x': ['2013-01-01', '2013-01-02', '2013-01-03'],
'data2': [10],
'data3': [10, 20],
'data1': [10, 20, 30]
}
},
axis: {
x: {
type: 'timeseries',
tick: {
format: '%Y-%m-%d'
}
}
}
});

```