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  • Tammy LaBiche-Henley thinks this is interesting:

Objects that are of similar color, shape, size, or orientation are perceived as related or belonging to part of a group. In Figure 3.3, you naturally associate the blue circles together on the left or the grey squares together on the right.

From

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

Note

Similarity - things of the same color, shape, size, etc belong to the same group or are related