highlightValue(x: Double, dataSetIndex: Int, stackIndex: Int) What does stackindex mean?

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## What did you do?

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## What did you expect to happen?

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## What happened instead?

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## Charts Environment

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## Demo Project

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Start by locating the Swift declaration and documentation for highlightValue(x: Double, dataSetIndex: Int, stackIndex: Int). Check how stacked chart entries are represented and where this API is documented; done means the meaning and valid use of stackIndex are clearly explained.

Written by the indexing model from the issue text.