I mean, imagine being a parent who is only eating a plant based diet, buying green energy, not flying at all and living car free. Somehow you are assuming that your children will violate all of these rules despite living in the same household as you. They somehow eat meat, they somehow fail to use the green energy you bought for them and they go on transatlantic flights on their own and they even have a damn car! By definition no matter what you do for yourself is going to matter because you are creating a CO2 spewing monster according to this graph. Is that really what happens in practice? I doubt that. It is far more likely that your child has learned some CO2 saving measures from you and therefore when you are having one child less you aren't eliminating the CO2 monster, you're eliminating a CO2 saver.

There is also another more obvious issue with the graph. It is double counting emissions by counting your children's emissions as your own. Meaning there is no real upper bound to how much CO2 you save. If humanity colonizes the entire universe the impact of your decision to not have one child would be literally astronomical. Your decision could have avoided hundreds of trillions of lives being brought into existence or even infinite if all humans die without having children. Just think of the CO2 savings!

Because the last section of the graph is highly misleading I hope nobody bases their decision to have children on it. If you really want to have less children then first consider whether your child will be a CO2 monster or not.