clear; 
clf;  
n=500; 
a=6*pi; 
freq = 20; 

delta = a/n;  
b=n+1; 
hold off;
numframes = floor(b/freq)+1 
M = moviein(numframes); 
for k =1:b
    t(k)=delta*(k-1);  
    
    x(k)=sin(t(k))-2*cos(2*t(k)); 
end;
 
t=0:delta:a;
x=sin(t)-2*cos(2*t);

ybottom = min(x)
ytop = max(x);
xright = max(t);
xleft =  min(t);
axis([xleft xright ybottom ytop])
grid on  

ezplot('t','sin(t)-2*cos(2*t)',[0,6*pi]);hold on;

marker1 = line('color','g','Marker','.','markersize',40,'EraseMode','xor','xdata',t(1),'ydata',x(1));
graph1=line('color','r','Linestyle','-','erase','none','xdata',[],'ydata',[]);

j=0;  
for k=1:b  
  X1(k)=t(k);
  Y1(k)=x(k);
    if(floor(k/freq)*freq == k) 
        set(graph1,'Xdata',[X1],'Ydata',[Y1]);
        set(marker1,'XData',X1(k),'YData',Y1(k));
        j=j+1
        M(j)=getframe;
    end;     
end
set(graph1,'Xdata',[X1],'Ydata',[Y1]); 
set(marker1,'XData',X1(b),'YData',Y1(b));
M(numframes)=getframe

movie(M,10);