Computer-based visualization systems provide visual representations of datasets designed to help people carry out tasks more effectively. Visualization is suitable when there is a need to augment human capabilities rather than replace people with computational decision-making methods. The design space of possible vis idioms is huge, and includes the considerations of both how to create and how to interact with visual representations. Vis design is full of tradeoffs, and most possibilities in the design space are ineffective for a particular task, so validating the effectiveness of a design is both necessary and difficult. Vis designers must take into account three very different kinds of resource limitations: those of computers, of humans, and of displays. Vis usage can be analyzed in terms of why the user needs it, what data is shown, and how the idiom is designed. Tamara will discuss the implications of all this trickiness for systems visualization, where the datasets include trace logs, network traffic, and semi-structured text in addition to the classic big table of numbers. One good way forward is to think hard about how to transform your original data into a form that's well suited for addressing the user's problems before you dive into the details of exactly how to draw any pictures.