Find Elapsed Time on Serial Plotter
It depends on how precise you want your time-base to be. The plotter's x-axis is in sample-count, not time, so you have to emit samples at a fixed rate for it to have time-significance.

Your

loop(){
   yadda();
   zadda();
}


may or may not run at a constant rate, depending on what takes place in each iteration of the loop, as well as on your definition of "constant" (I don't believe I just said that...) 

Even if none of your code is conditional, binary to ASCII conversion, clock and Serial interrupts, and other low-level code we don't see, may run slightly differently from iteration to iteration; e.g. the number of interrupts during any given iteration may well vary.

If you need to hold a tighter timebase, stalling at the bottom of the loop until millis() reaches (the previous millis() + an interval constant) will at least get you to within a millisecond of a fixed loop-rate. You could, of course, do the same thing with micros(), as long as you know for a fact that the loop can never take longer to execute than 1/frequency you need it to run at, and even then, it should take a fair slop-factor (that's a technical term! :) less than that, "just in case".

Update:

Though I wrote "stalling" at the end of the loop, a better technique is to call every job (in your case, 1 of them - for now ? ) as frequently as possible and let each job decide whether it needs to execute. So in your case:

loop {
  maybeSampleAndPlot();
  // maybeDoSomeOtherThing();  // not implemented yet...
}

maybeSampleAndPlot(){
  time = millis();
  if time - previousSampleTime &gt;= MSEC_040
    save (time + MSEC_040) as nextSampleTime;
    sample the data;
    plot the data;
  endif
}