Various tools -- notably MGM and gkrellm -- provide simple languages for describing how to display system statistics. These are typically fairly limited, though, and only worth within the confines of that application.
I'd like a general declarative language for visualising real-time data: not just system statistics, but anything from room temperature to stock prices. I could then build a panel application that took a load of data sources and a description of their visualisations, and displayed them. New visualisations could be written in terms of basic graphics and mathematical relationships.