Hi Georgina,
In my understanding, you want to multiply the y-values of a particular plot by a constant factor so that the entire plot scales up. This can be achieved by getting the handle to the required plot and changing the property YData. For example, if you need to double the amplitude of the second plot, you can try the following:
VAR= rand(2000,4);
sepa=5;
for i=1:4
p(i) = plot(VAR(:,i)+sepa*(i-1));
hold on
end
p(2).YData = 2*p(2).YData;
