class_docstr: Metaclass for PlotlyCallback classes.
  
  We want each callback class to keep track of all of the instances of the class.
  Using a meta class here lets us keep the logic for instance tracking in one place.
code: def __init__(cls, name, bases, attrs):
      super().__init__(name, bases, attrs)
  
      # Create weak-value dictionary to hold instances of the class
      cls.instances = WeakValueDictionary()
  
imports: from weakref import WeakValueDictionary
  from ...element import Tiles
  from ...streams import BoundsX, BoundsXY, BoundsY, RangeX, RangeXY, RangeY, Selection1D, SelectionXY, Stream
  from .util import _trace_to_subplot
label_desc: Use image node_id 1 to create a new PlotlyCallbackMetaClass object from inherited base classes: type with example: obj = PlotlyCallbackMetaClass(cls, name, bases, attrs)