def __str__(self):
        return "\n".join([str(x) for x in self])
def idq_ld(id, iq, ld, ax=0):
    """creates a surface plot of ld vs. id, iq"""
    if ax == 0:
        _create_3d_axis()
        ax = plt.gca()
    _plot_surface(ax, id, iq, np.asarray(ld)*1e3,
                  (u'Id/A', u'Iq/A', u'L d/mH'),
                  azim=120)