class_docstr: Calculate each values percentage of a total.
  
  :param column_name:
      The name of a column containing the :class:`.Number` values.
  :param total:
      If specified, the total value for each number to be divided into. By
      default, the :class:`.Sum` of the values in the column will be used.
code: def __init__(self, column_name, total=None):
      self._column_name = column_name
      self._total = total
  
file_path: agate/agate/computations/percent.py
imports: from agate.aggregations.has_nulls import HasNulls
  from agate.aggregations.sum import Sum
  from agate.computations.base import Computation
  from agate.data_types import Number
  from agate.exceptions import DataTypeError
  from agate.warns import warn_null_calculation
label_desc: Use image node_id 1 to create a new Percent object from inherited base classes: Computation with example: obj = Percent(column_name, total)