In my "Introduction to LabVIEW" talk, I have a slide titled "Concept of Time", which points out that Time is an important data feature of LabVIEW (think of digital samplings at evenly-spaced time intervals, or sampling at a particular frequency (1/time)). LabVIEW has a data-type, the Waveform, that bundles time-of-acquisition (t0), time-interval-of-sampling (dt), and the sampled data array (Y) in a cluster -- it is the optimal data structure to save sampled data from a DAQ device. If you pass a Waveform (which includes dt) to an FFT routine, it will give you a Cluster that has an array of frequencies and also the frequency spacing, df.
Look at the Waveform Functions on the Block Diagram. There's one called "Build Waveform". Expand it to show t0 and Y. Y you wire in from the 2D Dbl that you currently have going into your Spectral Measurements Express VI. dt you compute from 1/Sampling Frequency. In fact, if you do this a step earlier when you are parsing your XML string and get an Array of Waveforms, your Waveform Graph will have the correct Time axis (because you gave it dt).
Bob Schor
