So, here is the deal with chocolate...it is ALL about the TYPE of chocolate, and yes, to a certain degree it will affect others more adversely than others based on tolerance and size.
The issue is a compound call Theobromine that is contained in chocolate. Dark chocolate has SIGNIFICANTLY more than semi sweet or
milk chocolate - the stuff on your commercial chocolate bar has even less...
The second big issue is size, as there is a finite line between poisoning and tolerance.
Theobromine affects the heart, lungs and nervous system, and also causes dehydration. 100mg-500mg per kg of body weight is the toxic level of Theobromine; 50mg - 225mg per pound of dog.
Ideally, if dog ingests chocolate (check with your vet) force feed salted water to induce vomiting to try to remove as much of the chocolate before it is digested. Do this in the bathtub so vomit can be flushed immediately and little clean up is required.
Commercial chocolate bar and a large dog, unlikely to be an issue; Chocolate covered cherries, unlikely to cause issue. Fancy, solid, dark chocolate such as Truffles, you are likely in the danger zone, DEPENDING on quantity eaten and size of dog. Generally a Google search will bring up a chocolate calculator that compares your dogs weight and the specific type of chocolate to determine IF levels are dangerous.
Grapes: Issue here is it is CERTAIN dogs and CERTAIN grapes - but this has not (to my knowledge) been clearly ascertained at this time.
What we do know is that SOME dogs, upon ingesting grapes or raisins suffer rapid, generally, untreatable kidney failure.
There are SIGNIFICANT variances as to tolerance - one dog is fine the other outright dies...so it is now recommended to disallow anything in the grape family (currants, raisins etc) as this issue is NOT currently well understood.