def plot_line(ax,projection,x,y,z,scales=(1.,1.,1.),line_args={}):
    """Support plotting a line.

    Args:
        ax: axis to be plotted.
        projection (str): support *3d, xy, xz, yz*.
        x (float): x coordinate.
        y (float): y coordinate.
        z (float): z coordinate.
        scales (tuple of float): axis scales.
        line_args (dic): matplotlib args for line plot.

    """

    x = x / scales[0]
    y = y / scales[1]
    z = z / scales[2]

    if projection=='3d':
        pl = ax.plot(x,y,z,**line_args)
    else:
        if projection=='xy':
            _x, _y = x, y
        elif projection=='xz':
            _x, _y = x, z
        else:
            _x, _y = y, z

        pl = ax.plot(_x,_y,**line_args)

    return pl