class_docstr: Class for creating and modifying the causal graph.
  
  Accepts a networkx DiGraph, a :py:class:`ProbabilisticCausalModel <dowhy.gcm.ProbabilisticCausalModel`, a graph string (or a text file) in gml format (preferred) or dot format. Graphviz-like attributes can be set for edges and nodes. E.g. style="dashed" as an edge attribute ensures that the edge is drawn with a dashed line.
  
   If a graph string is not given, names of treatment, outcome, and confounders, instruments and effect modifiers (if any) can be provided to create the graph.
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code: def all_observed(self, node_names):
      for node_name in node_names:
          if self._graph.nodes[node_name]["observed"] != "yes":
              return False
  
      return True
  
imports: import itertools
  import logging
  import re
  import networkx
  from dowhy.gcm.causal_models import ProbabilisticCausalModel
  from dowhy.utils.api import parse_state
  from dowhy.utils.graph_operations import daggity_to_dot
  from dowhy.utils.plotting import plot
label_desc: Use image node_id 19 for calling the CausalGraph obj's underlying member method code with example usage: obj.all_observed(node_names) and returns: True, False