Tooltips show on wrong height when grouping after .flow()

There seems to be a problem with tooltips showing on hovering when first flowing data to a chart, followed by grouping it all together. Tooltips show up on the original heights of the data points, not the ones after grouping.

jsFiddle here: http://jsfiddle.net/4kL1xjt5/

Example code (same as in jsFiddle):
```javascript
var chart = c3.generate({
data: {
x: 'x',
columns: [
['x', '2012-12-29', '2012-12-30', '2012-12-31'],
['data1', 230, 300, 330],
['data2', 190, 230, 200],
['data3', 90, 130, 180],
]
},
axis: {
x: {
type: 'timeseries',
tick: {
format: '%m/%d',
}
}
}
});

setTimeout(function () {
chart.flow({
columns: [
['x', '2013-01-11', '2013-01-21'],
['data1', 500, 200],
['data2', 100, 300],
['data3', 200, 120],
],
duration: 1500,
done: function () {
chart.groups([["data1", "data2", "data3"]]);
},
});
}, 1000);
```

* **C3 version**: 0.6.6
* **D3 version**: 5.5.0
* **Browser**: Firefox 60.0.2 (64-bit)
* **OS**: Fedora 28