tooltip.formatter is missing information how to format dataset named fields using string formatting

[KrzysztofMadejski]

There is explanation how to pick different fields from the data object, ie. c0, c1.

I'd like to use named fields (when data point is an object), I was expecting ccount or c.count to work, but it doesn't work. I wonder if it's supported and if so how it can be expressed in a string template?