Explanation of parametric axes scales

@dberlow as we discussed today, I've been chatting with @rsheeter about parametric axes again recently.

Please could you explain both the idea of why parametric axes are defined in an absolute scale, why the scale is per mille, and, what harm would be caused by defining the axes in other ways?

No file, test, or documentation entry point is named. Start by locating the project material that defines parametric axes, then research why their values use an absolute per-mille scale and what problems alternative definitions could cause. Done means a clear explanation addresses all three questions and is added to the appropriate documentation.

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