Software tools that enable interactive data visualization are now commonly available for engineering design. These tools allow engineers to inspect, filter, and select promising alternatives from large multivariate design spaces based upon an examination of the tradeoffs between multiple objectives. There are two general approaches for visually representing data: (1) discretely, by plotting a sample of designs as distinct points; and (2) continuously, by plotting the functional relationships between design variables and design metrics as curves or surfaces. In this paper, we examine these two approaches through a human subjects experiment. Participants were asked to complete two design tasks with an interactive visualization tool: one by using a sample of discrete designs and one by using a continuous representation of the design space. Metrics describing the optimality of the design outcomes, the usage of different graphics, and the task workload were quantified by mouse tracking, user process descriptions, and analysis of the selected designs. The results indicate that users had more difficultly in selecting multiobjective optimal designs with common continuous graphics than with discrete graphics. The findings suggest that innovative features and additional usability studies are required in order for continuous trade space visualization tools to achieve their full potential.