def add_node(self, node_id, title):
    """
    Add a node to the network.

    Parameters
    ----------
    node_id : str, or int
        A  unique identifier for the node, typically the zettel ID.
    title : str
        The text label for each node, typically the zettel title.

    """
    self.graph.add_node(node_id, title=title)