Fix bug in mapData: use updated index provided by forEachData

I updated the offset provided by forEachData but forgot to
remove the decrement in mapData(), resulting in the first
element being stored at offset -1 and the last element of
the array being undefined.

This bug affected all the layouts using mapData:
* group by Country
* group by Country Aggregation
* group by Country Aggregation > WG
* group by Country Aggregation > Role
* group by Institution Type > WG