In fact, I made a copy of voltage-input.rrd as voltage-output.rrd, and the AC voltage part of the graphs started to work:
And after cloning charge to temperature-battery, temperature-ups, frequency-input and frequency-output, those graphs started to be shown (with the fake data).
So, the broken graphs are because the missing .rrd databases due parameters/reports not supported by some UPS. IMHO the developers should consider this.
In the meantime, how could I set not to include the missing voltage-output.rrd in order to display the graph for the existing voltage-input.rrd? (I found apcups.js and nut.js files, but don't know whether it would be the right files to modify).