When you say you fill your time data array dynamically, do you mean the length of the array change?
The reason I am asking is that the behavior you describe is "weird". The only "thing" that can make your Y-values grow is if the energy in each bin grows. This can happen if: 1 - You bin size gets reduced, that is the ratio "Sample Rate / number of points in the array" increases. So your array length is not constant 2 - Your overall signal energy increases. Are you averaging and if yes how? Please post your data so we can look at them.
