You are losing all timing information that was present in the waveform when you go and build it into a double array. So whatever your intial time was and your delta Time get replaced with zero (roughly midnight Jan 1, 1904 GMT) and a delta T of 1 second.
For your XY graphs, your X values start at zero and advance by 1 every 30 seconds.
You may want to consider using a waveform graph and use the function Append Waveforms to store your data in the shift register. It will keep the timing and start time in thew waveform, and you won't have to mess with X and Y arrays.
