How to use the svglib.svglib.Box function in svglib

def test_percentage_conversion(self):
        "Test percentage length attribute conversion."

        ac = svglib.Svg2RlgAttributeConverter()
        ac.set_box(svglib.Box(0, 0, 50, 150))
        # Percentages depend on current viewport and type of attribute
        length = ac.convertLength("1e1%", attr_name='width')
        assert length == 5
        length = ac.convertLength("1e1%", attr_name='height')
        assert length == 15