from typing import Dict


def _new_ax_png_name(dct: Dict[str, str]) -> str:
    """
    Returns the first string of the form "axNNN.png" not present in dct, for NNN=000, 001, 002, ...
    """
    idx = 0
    while True:
        base = f"figure{idx:03d}.png"
        if base not in dct:
            return base
        idx += 1