UPSwatch is a diagnostic and monitoring utility that collects statistics on an uninterruptible power supply using rrdtool and NUT in order to monitor its performance over time. Output is displayed in graphs on a web page. The current version creates two pages — a "Day View" that displays graphs covering the last two hours and the last 24 hours, and a "Month View" with graphs covering the last seven and the last thirty days. (See sample two-hour graphs on this page. Most of the recent tuning I've been doing has been to remove coloring artifacts from the graphs, which unfortunately turns out to require being a little more wasteful and a little less clever about generating them.)
At this time, UPSwatch v1.0 is compatible only with NUT and is optimized for the HP R3000XR and similar UPS models using transformerless DSP technology with a line-voltage battery pack, although it may be modified in future to add simple customization options allowing use with transformer-based UPS mddels with 24V battery packs, and to work with apcupsd.
Download the current version of UPSwatch here. The tarball contains everything you should need except for NUT and rrdtool, including Gabor Kiss's modified Nut.pm module. At its present stage of development, you're somewhat on your own as far as installation and configuration, but I'll gladly assist with configuration questions via email.