Fog of war is a technique used in video games for nearly four decades–first and often seen in complex tactical and strategy games. Like many data visualization techniques found in video games, it hasn’t been adopted in more traditional data visualization. Here is a simple implementation. There are a set of points displayed on-screen with a set of hexes overlaid upon them. By clicking on a hex, you reveal what is underneath, and if there are no points underneath that hex, nearby empty hexes are also revealed. This hearkens to the mechanics of another, simpler game: minesweeper.
Other examples: