pro plottick, x, y, tick, color, _extra=extra

    zero = [0.,0.]
    for i=0, n_elements(x)-1 do begin
        oplot, x[i]+tick,y[i]+zero, color=color, _extra=extra
        oplot, x[i]+zero,y[i]-tick, color=color, _extra=extra
        oplot, x[i]-tick,y[i]+zero, color=color, _extra=extra
        oplot, x[i]+zero,y[i]+tick, color=color, _extra=extra
    end
    
end