We have been working on a unit system for end-user spreadsheets that is based on the concrete notion of units instead of the abstract concept of types. In previous work, we defined such a system formally. In this paper, we describe a visual system to support the formal reasoning in two ways. First, it supports communicating and explaining the unit inference process to users. Second and more important, our approach allows users to change the system?s reasoning by adding and customizing the system?s inference rules.