[Feature request] custom label_generator

[yunyuyuan]

Checklist

• I am running the latest version of the card
• I checked if there isn't another feature request opened with the same request
• I checked that the feature is not already available in the dev branch

For example, I created a RAM stack chart. To accumulate the graph, I need to sum the values. The chart itself is perfect, but it results in incorrect labels.If there is a label_generator function, that would be great.

[github-actions]

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