How to create a declining wall thermometer indicator bar?

I have a problem that I hope someone might be able to help with by pointing me to a page link or video tutorial.

I want to slowly zoom in on a traditional vertical wall type thermometer (see attached image), tilting down as the vertical indicator fluid bar moves down the scale to 0° freezing point and below. I can’t create the actual temperature reduction in real time. So I need to find a way to overlay a declining fluid bar to indicate the fall in temperature as I do the camera move down and in.

Although I have FCPX and Motion, I have no experience of such special effects, so all advice and suggestions welcome.

Thanks!