Can I show a string in the header?

[Nuuki9]

I'm creating some indoor air quality popups - here's an example for VOCs:

Alongside the raw value (507.6 in this case) I also want to show the rating. I have a 1-5 value for this, which is shown in the header (1 in this case), but I want to convert this to a string (from "Poor" to "Excellent"). I took a look at the transform option but this is for transforming to a value, not a string.

Am I missing something obvious or is this not possible?