Not sure if I understand your question: Do you have all t and y data available and want to get only the y data between two t values? Or do you need to extract the t and y data from a figure?
If first case:
tA=0;tB=1500;
yinterval=y(t>=tA & t<=tB);
If second case: Load the figure, then
hc=get(gca,'children');
data=get(hc,{'xdata','ydata'});
t=data{1};
y=data{2};
and proceed as in the first case.
