Returns a graph showing a solve failure.

        Args:
            failure_index: See `failure_reason`

        Returns:
            A pygraph.digraph object.

def get_fail_graph(self, failure_index=None):
        """Returns a graph showing a solve failure.

        Args:
            failure_index: See `failure_reason`

        Returns:
            A pygraph.digraph object.
        """
        phase, _ = self._get_failed_phase(failure_index)
        return phase.get_graph()