Sergey:
in the Waveforms toolbox, there is a "number of waveform samples" vi. You can read the entire number of samples in the entire waveform and then reiterate that to read that by some divisor. For example, if you have 5,000,000 points in the wave form you can index it by 5,000,000/x, for example.
Eric P. Nichols
P.O. Box 56235
North Pole, AK 99705