Set limits or all axis the same, if vmax not given, use [-vmin, vmin].

def xyzlim(vmin, vmax=None):
    """Set limits or all axis the same, if vmax not given, use [-vmin, vmin]."""
    if vmax is None:
        vmin, vmax = -vmin, vmin
    xlim(vmin, vmax)
    ylim(vmin, vmax)
    zlim(vmin, vmax)