0 is only relevant for linear utility/impact functions The effect of an isotope isn't necessarily linear in its magnitude from 0. The baseline should be something that matters, like maximum safe dose.
If you have a baseline and a scale that indicates an actual effect, then by all means use it. That’s not the case here, nor has it been the case for any other such graph I can remember seeing. Universally, the bottom is chosen as the minimum value in the data being plotted.
mikeash|7 years ago