88v would be well in the territory of a brownout, but according to that graph you are sitting comfortably within 10% of 115v, with one exception. This isn't bad, if you viewed the graph with the y axis range set from 0v-140v it would look nearly flat. I wouldn't worry about it, the UPS will handle the small transient stuff.
Electricians
You can usually adjust the sensitivity of the UPS, but the power supplies should be able to handle a broad range of input - to a degree at the expense of their longevity. It all runs in DC. If the UPS supplies more power, the battery will die slightly more quickly, but the battery will die pretty quickly anyways.
The best method is to measure voltage near the watt hour meter.
Maybe your home installation isn't good enough, like overloaded, too thin wires, etc.
Not sure how mains is distributed in the Canada. Too many users on the same transformer?
Do you have a second phase?
The UPS is connected to an outlet that is about 1 metre from the electrical breaker panel. It's a new house so the wiring should be OK. There are only 4 buildings/services on my local transformer so I think we're stressing that. I have not checked the other phase and other comments have suggested that the variation I'm seeing is within normal range, so I probably won't.