AC voltages always fluctuate, no matter what you do.. It would be much more helpful to use graph-mode or logging for something like that, so you might be able to see what happens on the grid and separate your own stuff from what your neighbor might be doing. But..

I mostly graph/log currents with mooshimeter, as there should not be anything interesting to see on mains voltage :D .. For AC i basically always use current transformers, so it is also much safer and easier.

If i do find some multimeter that offers to zero at mains voltage, i will mash it with a hammer, as it would be really dangerous. At least here in europe with 230/400v.