More than one way. The linked graph cites the paper it was generated in which has all the details https://agupubs.onlinelibrary.wiley.com/doi/epdf/10.1029/199...

The ratio of oxygen and hydrogen isotopes in water molecules.

Both elements have two abundant and stable isotopes: one heavier and one lighter.

The quotient of the heavier isotopes in land ice is a linear function of the temperature, because it takes more energy to evaporate the heavier isotopes from icean water.