Selection issue with overlapping series

If you display two series as:
- data1 is a bar, selectable
- data2 is a line, not selectable

When you hover a point data2 that overlaps data1 you see the point as being selectable (you have the tooltip of data2 and the 'pointer' cursor).

If you click that point, you have both the click event on data1 and data2.

Apparently #608 was not really fixed.

See: https://jsfiddle.net/mcd6ucge/24/

``` javascript
var chart = c3.generate({
data: {
columns: [
['data1', 100, 100],
['data2', 50, 150]
],
selection: {
enabled: true,
isselectable: function(d) {
return d.id === 'data1';
}
},
onclick: function(d) {
console.log('clicked', d.id);
},
type: 'bar',
types: {
data2: 'line',
}
},
tooltip: {
grouped: false
}
});

```