Get min and max values for axes and colorbar if not given

def check_minmax(plotman, cid, xmin, xmax, zmin, zmax, vmin, vmax):
    '''Get min and max values for axes and colorbar if not given
    '''
    if xmin is None:
        xmin = plotman.grid.grid['x'].min()
    if xmax is None:
        xmax = plotman.grid.grid['x'].max()
    if zmin is None:
        zmin = plotman.grid.grid['z'].min()
    if zmax is None:
        zmax = plotman.grid.grid['z'].max()
    if isinstance(cid, int):
            subdata = plotman.parman.parsets[cid]
    else:
            subdata = cid
    if vmin is None:
        vmin = subdata.min()
    if vmax is None:
        vmax = subdata.max()

    return xmin, xmax, zmin, zmax, vmin, vmax