[Tooltip] In timeseries, group mode requires mouse to be very close to a point to work

* **C3 version**:0.6.1
* **D3 version**:5.4.0
* **Browser**:NA
* **OS**:NA

While upgrading from = <0.4.20, 3.5.17> to <0.6.1, 5.4.0> I noticed that what I supposed being a feature is not available anymore.
In oldest release, in grouping mode, all Y data was available as selected input for tooltip even if mouse was not close to a point. Now, due to computing of euclidian distance, you need to be on a point or close to it to have this data.

In timeseries, overriding distance computing with a simple X axis absolute distance is enough to go back to previous behavior.