Calculates (x, y) (max, min) for a list of Spectrum objects.

    Returns (xmin, xmax, ymin, ymax, xspan, yspan)

def calc_max_min(ss):
    """"Calculates (x, y) (max, min) for a list of Spectrum objects.

    Returns (xmin, xmax, ymin, ymax, xspan, yspan)
    """
    xmin, xmax, ymin, ymax = 1e38, -1e38, 1e38, -1e38
    for s in ss:
        assert isinstance(s, ft.Spectrum)
        if len(s.x) > 0:
            xmin, xmax = min(min(s.x), xmin), max(max(s.x), xmax)
            ymin, ymax = min(min(s.y), ymin), max(max(s.y), ymax)
    xspan = xmax-xmin
    yspan = ymax - ymin
    return xmin, xmax, ymin, ymax, xspan, yspan