Piechart tooltip shows field names

[markov00]

Is your feature request related to a problem? Please describe.
In Kibana, on a piechart, the tooltip shows "field" and "value" as seen in the following screenshot

Describe the solution you'd like
Add an accessor to the Partition component to specify name/ids of values

Describe alternatives you've considered
We can find a better way to merge this request into an accessor type that takes into consideration: the value accessor, a value name, a value formatting function.

Additional context
from #532

Kibana Cross Issues
n/a

Checklist

• this request is checked against already existing requests