Is there a reason to include all the fine details? The details serve little purpose other than to shout at readers that there is a lot of data behind this chart. It is impossible to compare the different drugs on the individual harmful effects based on reading this chart. All we can see is the comparison of total harm. (By the way, I can't explain why anabolic steriods rated 10 would be sandwiched between khat and ectasy both rated 9.)
It turns out there is an easy way to fix this chart. We turn to the original Lancet paper which contained this chart, by David Nutt (link). The 16 categories of harm are nicely organized into a tree structure:
Instead of taking data from the right side of this tree, we can take data from the aggregated levels. The sacrifice in detail comes with a major benefit in clarity. In the original paper, Nutt produced a chart that aggregated everything to two levels:
It's amazing how much more we learn from this chart even though it has less data than the previous one. (I'd still remove the data labels since they are redundant when one has the axis labels.)
Similarly, we can plot a chart at the level of physical, psychological, social, etc., and it would still be much more readable than the "staggering" one.
PS. Apparently, David Nutt is a controversial character. See Wiki (link).