def __str__(self):
        return "\n".join([str(x) for x in self])
def i1beta_psid(i1, beta, psid, ax=0):
    """creates a surface plot of psid vs i1, beta"""
    if ax == 0:
        _create_3d_axis()
        ax = plt.gca()
    azim = -60
    if 0 < np.mean(beta) or -90 > np.mean(beta):
        azim = 60
    _plot_surface(ax, i1, beta, psid,
                  (u'I1/A', u'Beta/°', u'Psi d/Vs'),
                  azim=azim)