It is relatively simple to create an easy to update thermometer with two images and some CSS:
These are then overlaid an the backgrounds on two divs:
Adjust the slider to see the effect:
The thermometer image is editable as a SVG. Changing the colors or number of tick marks can be done in a text editor. To generate the PNGs, use Batik or Inkscape (webstart doesn't allow export).
The image has a very simple script embedded in it that makes it render differently with a ?full parameter.
