@staticmethod
def set_slope_class(elt, value):
    """ Set attribute class iaw positive or negative slope. """
    if abs(value) < 0.005:
        elt.attrib['class'] = 'stable'
    elif value > 0:
        elt.attrib['class'] = 'increase'
    else:
        elt.attrib['class'] = 'decrease'
