feat(data): Intent to ship data.stack.normalize.perGroup

[netil]

Issue

#4060

Details

Implement normalize per group option.

• Normalize per group (each group independently becomes 0-100%)
• Data not in any group will display with their original values.
  • If non-grouped data shares the same axis (e.g., y), it will be displayed on the 0-100 absolute scale. To display original values, assign them to a separate axis (e.g., y2).

data: {
    stack: {
      normalize: {
        perGroup: true
      }
    }
}

[coveralls]

coverage: 94.147% (-0.03%) from 94.175%
when pulling faeed95 on netil:normalizePerGroup#4060
into 76151ad on naver:master.