That chart is being misused. It's not which drugs are more harmful, it's which drugs are more harmful as they're currently used in our society
Prof Nutt told the BBC: "Overall, alcohol is the most harmful drug because it's so widely used.
If you were so inclined, you could read that as an endorsement of prohibition. Other drugs cause less harm because they are prohibited and therefore less widely used. Yay prohibition!
Ultimately it doesn't make sense to try to score drugs on a single axis of harm because the risks are so different and can be mitigated to a greater or lesser extent. For instance in this graph
GHB is ranked higher on the "harm to users" scale than Ecstasy.
This is because it's very easy to kill yourself with a GHB overdose and there is also slight addiction possibility, with withdrawal being potentially life threatening (as with alcohol). That sort of harm could easily be mitigated with the ending of prohibition and education about proper use. When used with due care, GHB is one of the least harmful recreational drugs out there.
OTOH, Ecstasy is a neurotoxic drug and brain damage to users will persist basically no matter what (although it can certainly be mitigated).
So which is the more harmful drug, GHB or Ecstasy? It's not a question that really makes sense, it depends on the context in which they are taken.