def call_tree(self):
        """Returns call tree."""
        call_tree = {'stack': 'base', 'sampleCount': 0, 'children': []}
        for stack, sample_count in self._stats.items():
            self._insert_stack(reversed(stack), sample_count, call_tree)
        self._fill_sample_count(call_tree)
        if not call_tree['children']:
            return {}
        return self._format_tree(
            call_tree['children'][0], call_tree['sampleCount'])
Returns call tree.