from typing import Dict, Any, List, Optional


def sankey_diagram(
    source_col: str,
    target_col: str,
    value_col: str,
    title: Optional[str] = None
) -> Dict[str, Any]:
    """Sankey Diagram (placeholder spec)."""
    return {
        "type": "sankey",
        "title": title or "Sankey Diagram",
        "encoding": {"source": source_col, "target": target_col, "value": value_col},
    }
