Visualisation is often presented as a means of simplifying information and helping people understand complex data. In this paper we describe the design, development and evaluation of an interactive visualisation for spreadsheet formulae (EQUS). The work is justified on the grounds that these are widely used tools for significant numerical processing and modeling, yet the formula developed can be easily misunderstood. The development process was one of iterative refinement engaging an initial target audience of mid-teen learners, involving re-design and formative evaluation. The resulting visualisation techniques have been found to be broadly relevant to spreadsheet users beyond the initial target audience. EQUS has since been developed as fully integrated plug-in for MS Excel.