say you have
x = [1 2 3 4];
y = [8 12 16];
then you can fit a polynomial to the region you have data for (the first three samples) using
p = polyfit(x(1:length(y)), y, 1);
and estimate all the samples using
y_est = polyval(p, x);
or just the ones you don't have data for using
y_est = polyval(p, x(length(y)+1:end));
