68-95-99.7 Rule
Dark blue is less than one standard deviation away from the mean. For the normal distribution, this accounts for about 68% of the set, while two standard deviations from the mean (medium and dark blue) account for about 95%, and three standard deviations (light, medium, and dark blue) account for about 99.7%.
Source
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"Standard deviation diagram."
https://en.wikipedia.org/wiki/File:Standard_deviation_diagram.svg
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