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.
docstr: Record that codepoint *glyph* is being typeset using font *font*.
docstr_tok: ["Record","that","codepoint","*","glyph","*","is","being","typeset","using","font","*","font","*","."]
code: def track_glyph(self, font, glyph):
      """Record that codepoint *glyph* is being typeset using font *font*."""
      self.used.setdefault(font.fname, set()).add(glyph)
  
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 3 for calling the CharacterTracker obj's underlying member method code with example usage: obj.track_glyph(font, glyph) without return types