Abstract models of visualisation have been demonstrated to be useful in analysing visualisation artifacts and, in some cases, automatically generating visualisations from model-based specifications plus design rules. However, many visualisation problems, especially in the early design stages, do not lend themselves to a simple and/or coherent formulation which can be readily captured with current visualisation models. In this talk I will present design-time constraints as a design representation technique which offers the prospect of capturing a specification without requiring commitment too early or too strongly to tentative hypotheses about the design. I will consider how design-time constraints might be applied to data visualisation design and illustrate such an application via the Representer system, a successor to the Icongrapher system which supports the specification and use of design constraints.