class_docstr: Sankey diagram.
  
    Sankey diagrams are a specific type of flow diagram, in which
    the width of the arrows is shown proportionally to the flow
    quantity.  They are typically used to visualize energy or
    material or cost transfers between processes.
    `Wikipedia (6/1/2011) <https://en.wikipedia.org/wiki/Sankey_diagram>`_
docstr: Return the codes and vertices for a rotated, scaled, and translated
  90 degree arc.
  
  Other Parameters
  ----------------
  quadrant : {0, 1, 2, 3}, default: 0
      Uses 0-based indexing (0, 1, 2, or 3).
  cw : bool, default: True
      If True, the arc vertices are produced clockwise; counter-clockwise
      otherwise.
  radius : float, default: 1
      The radius of the arc.
  center : (float, float), default: (0, 0)
      (x, y) tuple of the arc's center.
docstr_tok: ["Return","the","codes","and","vertices","for","a","rotated",",","scaled",",","and","translated","90","degree","arc",".","Other","Parameters","--","--","--","--","--","--","--","--","quadrant",":","{","0",",","1",",","2",",","3","}",",","default",":","0","Uses","0-based","indexing","(","0",",","1",",","2",",","or","3",")",".","cw",":","bool",",","default",":","True","If","True",",","the","arc","vertices","are","produced","clockwise",";","counter-clockwise","otherwise",".","radius",":","float",",","default",":","1","The","radius","of","the","arc",".","center",":","(","float",",","float",")",",","default",":","(","0",",","0",")","(","x",",","y",")","tuple","of","the","arc","'s","center","."]
code: def _arc(self, quadrant=0, cw=True, radius=1, center=(0, 0)):
      """
      Return the codes and vertices for a rotated, scaled, and translated
      90 degree arc.
  
      Other Parameters
      ----------------
      quadrant : {0, 1, 2, 3}, default: 0
          Uses 0-based indexing (0, 1, 2, or 3).
      cw : bool, default: True
          If True, the arc vertices are produced clockwise; counter-clockwise
          otherwise.
      radius : float, default: 1
          The radius of the arc.
      center : (float, float), default: (0, 0)
          (x, y) tuple of the arc's center.
      """
      # Note:  It would be possible to use matplotlib's transforms to rotate,
      # scale, and translate the arc, but since the angles are discrete,
      # it's just as easy and maybe more efficient to do it here.
      ARC_CODES = [
          Path.LINETO,
          Path.CURVE4,
          Path.CURVE4,
          Path.CURVE4,
          Path.CURVE4,
          Path.CURVE4,
          Path.CURVE4,
      ]
      # Vertices of a cubic Bezier curve approximating a 90 deg arc
      # These can be determined by Path.arc(0, 90).
      ARC_VERTICES = np.array(
          [
              [1.00000000e00, 0.00000000e00],
              [1.00000000e00, 2.65114773e-01],
              [8.94571235e-01, 5.19642327e-01],
              [7.07106781e-01, 7.07106781e-01],
              [5.19642327e-01, 8.94571235e-01],
              [2.65114773e-01, 1.00000000e00],
              # Insignificant
              # [6.12303177e-17, 1.00000000e+00]])
              [0.00000000e00, 1.00000000e00],
          ]
      )
      if quadrant in (0, 2):
          if cw:
              vertices = ARC_VERTICES
          else:
              vertices = ARC_VERTICES[:, ::-1]  # Swap x and y.
      else:  # 1, 3
          # Negate x.
          if cw:
              # Swap x and y.
              vertices = np.column_stack(
                  (-ARC_VERTICES[:, 1], ARC_VERTICES[:, 0])
              )
          else:
              vertices = np.column_stack(
                  (-ARC_VERTICES[:, 0], ARC_VERTICES[:, 1])
              )
      if quadrant > 1:
          radius = -radius  # Rotate 180 deg.
      return list(
          zip(
              ARC_CODES,
              radius * vertices
              + np.tile(center, (ARC_VERTICES.shape[0], 1)),
          )
      )
  
file_path: matplotlib/lib/matplotlib/sankey.py
imports: import logging
  from types import SimpleNamespace
  import numpy
  import matplotlib
  from matplotlib.path import Path
  from matplotlib.patches import PathPatch
  from matplotlib.transforms import Affine2D
  from matplotlib import _docstring
label_desc: Use image node_id 2 for calling the Sankey obj's underlying member method code with example usage: obj._arc(quadrant, cw, radius, center) and returns: list