nanoplots, custom values reference line and area

[EconomiCurtis]

It would be lovely to provide support for reference line and area values custom for each row.

Currently, we can provide a set value or range, or "mean"... "last". But there are many situations where a user may one one ref area for one value, and another reference line for another row, and no reference value for another.

The example in All the new features in {gt} 0.10.0 announcement post is a good example of where this feature would be useful. For example, we might want to show (1) a reference line for WBC of the low-danger value for WBC, (2) show a reference area for the healthy range for of values for Neutrophils, (3) the first value for Viral Range, and (4) perhaps nothing for DBIL.

I think you might offer the ability to create a default value for ref line or area, and the ability to override this by named rows...? Or some other method?