From Introduction to Economic Analysis by McAfee and Lewis. They write that:
the marginal value curve is the inverse of the demand function, where the demand function gives the quantity purchased at a given price. Formally, if $x(p)$ is the quantity a consumer buys at price $p$, then $v(x(p))=p$.
I'm having some trouble justifying this. $x(p)$ is a function of price yet $v(x)$ describes marginal utility (I'm guessing "marginal value" is synonymous with this), so I'm not sure how $x$ is the inverse function of $v$.