I have done similar things internally for the company i work for. I found using GD and such to be a bit rough around the edges. Since most of my stuff is being used by a small admin, and i don't have to worry too much about portability, i have been using SVG for the representation. It's just XML, and it scales well on to a handheld (if needed). I only have one publicly available SVG work here, but if you look at SVG file for figure 12-4 in the examples from SVG Essentials, you will see a nice thermometer example.
in reply to Donation Tracking "Thermometer"
from the frivolous to the serious