I found an answer for my problem. it is abit long way but works for me.
I get the axis data of the figure object and redraw it as a matlab figure.
   hfvt = fvtool(filterobj);
   s = get(hfvt);
   hchildren = s.Children;
   haxes = hchildren(strcmpi(get(hchildren,'type'),'axes'));
   hline = get(haxes,'children');
   x = get(hline,'XData');
   y = get(hline,'YData');
after this i can "plot (x,y)" and do whatever i can.

