'label reference position in transAxes.
get_label_transform() returns a transform of (transAxes+offset)'

def get_axislabel_pos_angle(self, axes):

    loc = self._loc
    (pos, angle_tangent) = dict(left=((0.0, 0.5), 90), right=((1.0, 0.5), 90), bottom=((0.5, 0.0), 0), top=((0.5, 1.0), 0))[loc]
    return (pos, angle_tangent)
