Option to specify dataset has been normalized

[benmccann]

The time scale can be made much more efficient if we know that the index values are unique, sorted, and consistent (i.e. each dataset / labels contain same values)

Would we want one option that specifies all of these conditions are true? Where would we want this option to live? At the top-level options since it refers to all datasets? Or at the scale-level since that's what it's being used? (and technically would only have to be true for the datasets plotted against that scale)

I was originally attempting to solve this via a new dataset format (#6696), but that would only solve the consistency and not the uniqueness or sorting constraint. Adding an option was also suggested in #6695 (comment)

[benmccann]

Closed by #6775