Use when a chart, map, dashboard, table, or display must carry more than two variables and the flat surface is running out of room — symptoms include fake 3-D or ornament standing in for data, one separate mark per variable, density so thin it invites suspicion, or uncertainty about which of Tufte's five envisioning methods (micro/macro, layering, small multiples, color, space-time narratives) the problem calls for.
