class_docstr: Use a function turn facet value into a label
  
  Parameters
  ----------
  func : callable
      Function to label an individual string
code: def __init__(self, func: Callable[[str], str]):
      self.func = func
  
imports: from __future__ import annotations
  import typing
  from abc import ABCMeta, abstractmethod
  from ..exceptions import PlotnineError
label_desc: Use image node_id 1 to create a new _function_labeller object from inherited base classes: _core_labeller with example: obj = _function_labeller(func)