This kind of chart should have its workings and data readily available. It appears the only deaths included for nuclear are from some cancer estimate which is not readily accessible. It is not clear what the others death count includes.

Here is the entire data download that is actually included for this chart:

  Entity,Code,Year, (deaths per TWh)
  Biomass,,2014,4.63
  Brown coal,,2014,32.72
  Coal,,2014,24.62
  Gas,,2014,2.821
  Nuclear,,2014,0.074
  Oil,,2014,18.43

With it lacking any discussion of what these quite mysterious counts mean - this is often taken as proof of safety. Yet Nuclears exceptional score could be transformed by the kind of catastrophic accident which nuclear is capable of and which by great expense and some luck has avoided to date.

For example if weather during Fukushimas accident had blown fallout over Tokyo, or if a reactor anywhere releases similar or greater amounts of fallout anywhere in the world where a large population center is downwind, many thousands of cancers are possible, with children disproportionately affected by radiation exposure. Large areas of land can be made uninhabitable to civilization for decades or more, and depending on future civilizations standards.