def get_index_from_filename(
        file_name: str
) -> str:
    """
    Returns the index of chart from a reproducible JSON filename.
    :param file_name: `str`
        The name of the file without parent path.
    :returns: `str`
        The index of the chart (e.g., 1) or an empty string.
    """
    assembled_index = ""
    for name_char in file_name.replace(".json", "")[::-1]:
        if name_char.isnumeric():
            assembled_index += name_char

        else:
            break

    return assembled_index[::-1]