Neoclassical economics
Neoclassical economics usually supplements or supplants discussion of marginal utility with indifference curves, which were originally derived as the level curves of utility functions, or can be produced without presumption of quantification, but are often simply treated as axiomatic. In the absence of complementarity of goods or services, diminishing marginal utility implies convexity of indifference curves, although such convexity would also follow from quasiconcavity of the utility function.