code: def on_draw_event(self, widget, ctx):
      # to be overwritten by GTK3Agg or GTK3Cairo
      pass
  
imports: import functools
  import logging
  import os
  from pathlib import Path
  import matplotlib
  from matplotlib import _api, backend_tools, cbook
  from matplotlib.backend_bases import ToolContainerBase, CloseEvent, KeyEvent, LocationEvent, MouseEvent, ResizeEvent
  from gi.repository import Gio, GLib, GObject, Gtk, Gdk
  from .None import _backend_gtk
  from ._backend_gtk import _BackendGTK, _FigureCanvasGTK, _FigureManagerGTK, _NavigationToolbar2GTK, TimerGTK
label_desc: Use image node_id 20 for calling the FigureCanvasGTK3 obj's underlying member method code with example usage: obj.on_draw_event(widget, ctx) without return types