You can always use ActiveX (this will be phased out in a future version of MATLAB) to build plots directly in your Excel file. However, this can be difficult to program and slow to execute. Since you are ultimately trying to import a video into Powerpoint, you might want to try considering using VideoWriter() and getframe() to capture plots as frames (images) and compiliing them into an AVI using writeVideo().
v = VideoWriter(filename,'Motion JPEG AVI');
for i=1:Number_of_Frames
plot(X,Y)
frame = getframe(gcf);
writeVideo(v,frame)
end