text: """
  Pie Chart
  ===============================================================================
  
  
  >>> directory = "/workspaces/techminer2/data/"
  >>> from techminer2.annual_indicators import annual_indicators
  >>> from techminer2._pie_chart import _pie_chart
  >>> file_name = "/workspaces/techminer2/sphinx/images/_pie_chart.png"
  >>> _pie_chart(
  ...     series=annual_indicators(directory).num_documents, 
  ...     darkness=annual_indicators(directory).global_citations,
  ... ).savefig(file_name)
  
  
  .. image:: images/_pie_chart.png
      :width: 700px
      :align: center
  
  
  """
  
  
  import matplotlib
  import matplotlib.pyplot as plt
  import numpy as np
  import pandas as pd
  
  TEXTLEN = 40
  
  
  def _pie_chart(
      series,
      darkness=None,
      cmap="Greys",
      figsize=(6, 6),
      fontsize=9,
      wedgeprops={
          "width": 0.6,
          "edgecolor": "k",
          "linewidth": 0.5,
          "linestyle": "-",
          "antialiased": True,
      },
  ):
      """Plot a pie chart."""
      darkness = series if darkness is None else darkness
  
      cmap = plt.cm.get_cmap(cmap)
      colors = [
          cmap(0.1 + 0.90 * (d - min(darkness)) / (max(darkness) - min(darkness)))
          for d in darkness
      ]
  
      matplotlib.rc("font", size=fontsize)
      fig = plt.Figure(figsize=figsize)
      ax = fig.subplots()
  
      ax.pie(
          x=series,
          labels=series.index,
          colors=colors,
          wedgeprops=wedgeprops,
      )
  
      fig.set_tight_layout(True)
  
      return fig
  