You can choose whatever you like for units of work. As long as you and your team agree upon the common way to estimate and measured them and use the same method during the whole project.

I now that this is the answer you are asking for so here are some examples (I have no preference).
- If you are used to function points analysis. Use this method see http://www.nesma.nl/english/howfpa.htm
- alternatively you can use your own metrics and use a combined method for estimating (e.g. planning poker http://www.planningpoker.com/detail.html)

Interesting post.

In the presented graphs, what are the units of work and how do you know when these are complete?

Regards,