Not sure if your x axis are real world units, or simply indexes. If they're indexes, simply refer to them:
y1 = y(1245);
y2 = y(4567);
If they're not indexes, but real world values, then you need to find the index that is closest to your desired x value. Once you have that index, you can extract the y value you want.
[~, index] = min(abs(x - 1245));
y1 = y(index);
