avoid Performance loss due to S-Function in Simulink

i need for visualisation a scope in simulink which plots not only the actual value, so i change the sfunxy to autoscale and create a additional patch command to slove my problem this works so far but i gets while running slower and slower, the tester needs for the first 10s less then 4s for 10-20s its allready 1 minute and so on i think this is because the plot hold all the data how i can switch this of? i need only the actual data... here my code changes in sfunxy.mat in function mdlupdate

   function sys=mdlUpdate(t,x,u,flag,ax,block,varargin)

% always return empty, there are no states...
%
sys = [];

%
% Locate the figure window associated with this block.  If it's not a valid
% handle (it may have been closed by the user), then return.
%
FigHandle=GetSfunXYFigure(block);
if ~ishandle(FigHandle),
   return
end
global traceData;
%
% Get UserData of the figure.
%
start=fix(u(2))-2;
 if start<1
     start=1;
 end
 ende=ceil(u(2))+2;
 if ende>length(traceData.data)
     ende= length(traceData.data);
 end
ud = get(FigHandle,'UserData');
set(FigHandle,'NextPlot','replace');
set(ud.XYAxes, ...
    'Visible','on',...
    'Xlim', [u(1)-1 u(1)+1],...
    'Ylim', [min(traceData.data(start:ende,6))-2, max(traceData.data(start:ende,7))-2],...
    'XGrid', 'on',...
    'YGrid','on');
set(ud.XYLine,...
    'Xdata',u(1),...
    'Ydata',u(2),...
    'MarkerSize',20,...
    'LineStyle','x',...
  'EraseMode', 'background');
set(ud.XYTitle,'String','X Y Plot');
set(ud.XYAxes, 'NextPlot', 'add');

patch('Parent', ud.XYAxes,...
      'XData',[traceData.data(start:ende,1);traceData.data(ende:-1:start,1)]',...
      'YData',[traceData.data(start:ende,6);traceData.data(ende:-1:start,7)]',...
      'FaceColor', [0, 1, 0], ...
      'LineStyle','none',...
      'FaceAlpha', 0.2);
set(FigHandle,'UserData',ud);
drawnow