class_docstr: Helper for font subsetting by the pdf and ps backends.
  
  Maintains a mapping of font paths to the set of character codepoints that
  are being used from that font.
code: def __init__(self):
      self.used = {}
  
imports: from io import BytesIO
  import functools
  from fontTools import subset
  import matplotlib
  from ..None import font_manager, ft2font
  from .._afm import AFM
  from ..backend_bases import RendererBase
label_desc: Use image node_id 1 to create a new CharacterTracker object  with example: obj = CharacterTracker()