you'd do something like this where you would use fill to generate the ranges of the distribution
t = 0:.1:10;
x = 2*t.^2;
ulimit= 10*rand(size(x));
llimit= 10*rand(size(x));
hold on
h(1)=fill([t fliplr(t)],[x+3*ulimit fliplr(x-3*llimit)],'g','edgecolor','none')
h(2)=fill([t fliplr(t)],[x+2*ulimit fliplr(x-2*llimit)],'b','edgecolor','none')
h(3)=fill([t fliplr(t)],[x+ulimit fliplr(x-llimit)],'r','edgecolor','none')
set(h,'facealpha',.5)
plot(t,x,'b');
