ranges and each box is left-aligned within its subspace.
      Otherwise (*total* is *None*), *sep* must be provided and each box is
      left-aligned in its subspace of width ``(max(widths) + sep)``. The
      total width is then calculated to be ``N * (max(widths) + sep)``.

    Parameters
    ----------
    widths : list of float
        Widths of boxes to be packed.
    total : float or None
        Intended total length. *None* if not used.
    sep : float or None
        Spacing between boxes.
    mode : {'fixed', 'expand', 'equal'}
        The packing mode.

    Returns
    -------
    total : float
        The total width needed to accommodate the laid out boxes.
    offsets : array of float
        The left offsets of the boxes.
    )