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  • Tammy LaBiche-Henley thinks this is interesting:

We think of objects that are physically enclosed together as belonging to part of a group. It doesn’t take a very strong enclosure to do this: light background shading is often enough, as demonstrated in Figure 3.5.

From

Cover of Storytelling with Data: A Data Visualization Guide for Business Professionals

Note

Enclosure - things enclosed together belong to the same group