Circle Packing
Circle packing, also known as a circular tree map, is a variation of a tree map. It is thus a tool used to represent hierarchy and part-to-whole relationships. The primary difference between circle packing and a tree map is the use of circles to represent categories as opposed to squares. Circles are contained inside one another to display hierarchy. Similar to the tree map, the size of the circle represents quantity, while colors can be added to represent different categories.
Circle packing can be used to show organizational structure, for example, with circle area used to represent the number of employees and colors to represent different departments. However, circle packing is not as space efficient as tree maps or tree diagrams due to empty space between circles.
Image source: Mike Bostock