Oh! The "Stackable" option in the poll means "Stack in one position"?
I should choose "all permanents (non-stackable)", but I wrongly clicked "all permanents. (stackable)"......
Also, in my opinion, if it is untargetable, the poison should be 2|3. (I think 2|4 is too high.)
But if it is targetable, the poison should be 4|6 (since its cost is higher than the death spell "Poison").
I don't like the idea that upgraded card doubles the poison damage, unless the cost is also increased by 1.