These are actual concentrations of helium measured from the Fenton Hill core used to compute diffusivity.
The diffusivity for the red curve (the uniformitarian model) was computed by dividing the difference between the theoretical amount of helium produced over the conventional age of the rock and the measured amount remaining by the conventional 1.5-billion-year age of the rock.
However, the rate at which the gases can escape is highly dependent on the temperature of the minerals.
For example, the rate of helium diffusion at the hot temperatures 15,000 feet below the surface is about 160 times faster than the rate at the cooler temperatures at 4,000 feet.
This young age agrees with a literal reading of Scripture, but is at variance with the billions of years conventionally held.