OK. thanks. I am not unfamiliar with that "/" notation but it seemed strange to me to plot time vs time so I was wondering if the units were quotients. I appreciate the explanation but I am still lost. What is a "timestep"? The interval between measurements? If so, why would that vary significantly as a function of time? If not, what is a "timestep"? Google isn't much help -- Wiktionary= "an interval". I do get the point abut the calibration run.
ETA: It's been a while since I've been in a traditional measurement lab situation but last time I was, the interval between samples of data was determined by a crystal-based clock and didn't vary. So I'm still feeling like a dummy here because I am obviously missing something. That expanded plot means what? I hope I'm not the only one in the dark here.