class_docstr: Stage allows you evaluating mapping at more than one stage
  
  You can evaluate an expression of a variable in a dataframe, and
  later evaluate an expression that modifies the values mapped to
  the scale.
  
  Parameters
  ----------
  start : str | array_like | scalar
      Aesthetic expression using primary variables from the layer
      data.
  after_stat : str
      Aesthetic expression using variables calculated by the stat.
  after_scale : str
      Aesthetic expression using aesthetics of the layer.
class_docstr_tok: ["Stage","allows","you","evaluating","mapping","at","more","than","one","stage","You","can","evaluate","an","expression","of","a","variable","in","a","dataframe",",","and","later","evaluate","an","expression","that","modifies","the","values","mapped","to","the","scale",".","Parameters","--","--","--","--","--","start",":","str","|","array_like","|","scalar","Aesthetic","expression","using","primary","variables","from","the","layer","data",".","after_stat",":","str","Aesthetic","expression","using","variables","calculated","by","the","stat",".","after_scale",":","str","Aesthetic","expression","using","aesthetics","of","the","layer","."]
code: def __init__(self, start=None, after_stat=None, after_scale=None):
      self.start = start
      self.after_stat = after_stat
      self.after_scale = after_scale
  
file_path: plotnine/plotnine/mapping/evaluation.py
imports: from __future__ import annotations
  import numbers
  import typing
  import numpy
  import pandas
  import pandas.api.types
  from ..exceptions import PlotnineError
label_desc: Use image node_id 1 to create a new stage object  with example: obj = stage(start, after_stat, after_scale)