Revise to stack or not to stack timeseries in a panel

[consideRatio]

From #90 (comment), written by Yuvi:

To stack or not to stack? We previously had a haphazard approach where the answer was 'lolmaybe'. Be consistent.

From #90 (comment), written by Georgiana:

I consider the stacking confusing to be honest, and if I were to choose I'd not use it, but if this is something widely used in the scientific world then it's ok.

From #90 (comment):