class_docstr: Computes a Sankey diagram from a Graph element for internal use in
  the Sankey element constructor.
  
  Adapted from d3-sankey under BSD-3 license.
  
  Source: https://github.com/d3/d3-sankey/tree/v0.12.3
class_docstr_tok: ["Computes","a","Sankey","diagram","from","a","Graph","element","for","internal","use","in","the","Sankey","element","constructor",".","Adapted","from","d3-sankey","under","BSD-3","license",".","Source",":","https",":","\/\/github.com\/d3\/d3-sankey\/tree\/v0.12.3"]
code: def sourceTop(cls, source, target, py):
      y = target["y0"] - (len(target["targetLinks"]) - 1) * py / 2
      for link in target["targetLinks"]:
          if link["source"] is source:
              break
          y += link["width"] + py
      for link in source["sourceLinks"]:
          if link["target"] is target:
              break
          y -= link["width"]
      return y
  
imports: import math
  from functools import cmp_to_key
  from itertools import cycle
  import numpy
  import param
  from ..core.data import Dataset
  from ..core.dimension import Dimension
  from ..core.operation import Operation
  from ..core.util import get_param_values, unique_array
  from .graphs import EdgePaths, Graph, Nodes
  from .util import quadratic_bezier
label_desc: Use image node_id 13 for calling the _layout_sankey obj's underlying member method code with example usage: obj.sourceTop(cls, source, target, py) and returns: y