mehmet - every time you call plot you create a new graphics object, so you could just re-use a single plot object on every iteration of the loop. You seem to have some of the code already there just commented out and incorrectly referencing the (suppposed) figure handle when instead it should be the handle to the plot. For example,
close all, format long;
time = linspace(0,100,100);
x = rand(100,1);
a = NaN(size(time)); % <---- pre-size the a and b arrays
b = NaN(size(time));
hFig = figure(1)
hPlot = plot(a, b, 'r'); % <---- plot the data
axis([0 100 0 1.2])
grid on;
for i = 1:100
a(i) = time(i);
b(i) = x(i);
set(hPlot, 'XData', a, 'YData', b);
pause(0.001)
end
