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  • Tammy LaBiche-Henley thinks this is interesting:

We tend to think of objects that are physically close together as belonging to part of a group. The proximity principle is demonstrated in Figure 3.1: you naturally see the dots as three distinct groups because of their relative proximity to each other.

From

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

Note

Proximity - things close together are naturally a group