You could use animation frame to do this mixed with instance variable. Add a variable which is called "NumberOfItems" so if you push + you increate this number and if you push - you decrease it. Then, depending on the number you show the exact frame number relative to this number.
This would work if you have a max number of items to be stackable.
However, if you really need a new object. Then still use instance variables.. if your object have NumberOfItems lets say 2 then the next object created Y position will be Object.Height * NumberOfItems (because you already have 2 objects, if your object height is 10 then the result will be 20) + Object.Height (because the base coordinates are top-left, you haveto include the object height you want to add, so now the result will be 30 for the Y position).
This formulea doesn't take spacing between objects into consideration but this could easily be added.