Use centroid instead of center when positioning tooltips using `tooltip.trigger: "selection"`

[rarylson]

Use centroid (equivalent to center of mass) instead of center (geometrical center) when positioning tooltips using tooltip.trigger: "selection".

Google Charts uses this behavior, as showed in the images bellow, when centering the Slavbard country (its islands doesn't drastically change the tooltip position as using the geometrical center does).