It is possible to have "conditional legend items"? For (a made-up) example: Suppose I have a set of data-driven maps of a city's streets, with a normal set of street-related symbols such as freeways, major streets, residential streets, etc. On maps that have freeways, I want to show the freeway symbol in the map's legend; on maps that don't show any freeways, I don't want the freeway symbol to display.
