docs: def function_dependency_graph(self, func):
          """
          Get a dependency graph for the function `func`.
  
          :param func:    The Function object in CFG.function_manager.
          :returns:       A networkx.DiGraph instance.
          """
  
          if self._function_data_dependencies is None:
              self._build_function_dependency_graphs()
  
          if func in self._function_data_dependencies:
              return self._function_data_dependencies[func]
  
          # Not found
          return None