code: def downgrade(viz_type: str) -> None:
      """Downgrade a viz to the previous version."""
      migrate(VizType(viz_type), is_downgrade=True)
  
file_path: superset/superset/cli/viz_migrations.py
imports: from enum import Enum
  import click
  from click_option_group import optgroup, RequiredMutuallyExclusiveOptionGroup
  from flask.cli import with_appcontext
  from superset import db
label_desc: Use image node_id 3 for calling a global function with example usage: downgrade(viz_type) without return types