emi_ores-1.0+1.21.1+neoforge

I assume the Nether just goes from y-0 to y- 124 right?
So why is this graph showing a negative Y level range?

This is correct as per Occultism's data. https://github.com/klikli-dev/occultism/blob/9ef9f9611fbd06328fc375bcc940f89f97082ffa/src/generated/resources/data/occultism/worldgen/placed_feature/ore_iesnium.json#L13

When generating ores, if a y-value is picked that is outside of the world limits, it is skipped. So the net effect is that this ore generates more frequently the further down you go.