Well, fortunately, I solved my problem today. I'm extremely happy
The point is that I live in Russian Federation, and I have 220-230V, but the author of the script made the maximum value of 150 in RRD!..
(so I couldn't just figure out why there were NaN's in RRD while the script works quite well..)
I just changed the maximum value in an existing rrd file with the help of `rrdtool tune` (smth like `rrdtool tune <rrdfile> --maximum statistic:U`).
But if you just install it for the very first time, you can just modify the xml file: change <rrd_maximum> to the value you need (or just U).
Also check out <upper_limit> right there - this is the maximum value for the graph itself. It also should be modified.
That is all because I started reading docs about RRD while working on quite different thing. And then I remembered about the voltage...