Why does my MATLAB plot refresh at unpredicatable times even though 'EraseMode' is set to 'None'?

Why does my MATLAB plot refresh at unpredictable times even though 'EraseMode' is set to 'None'?   For example:   imax=100; map = jet(imax); ph = plot3(1,0,0,'b.','EraseMode','none','MarkerSize',8); axis([-2 2 -2 2 0 1]) axis('square') xlabel('x') ylabel('y') for i =1:imax, set(ph,'xdata',cos(2*pi*i/imax),'ydata',sin(2*pi*i/imax),'zdata',i/imax,'markerEdgeColor',map(i,:)); % pause drawnow end The plot should show a continuously growing spiral, but instead it refreshes at random points in the loop. This happens even when the "pause" statement is uncommented.