I read the chapter on Sparklines, and I couldn't figure out one thing: In those mini line graphs (stocks, glucose levels, etc) where's the zero line? Without some scaling reference, those little stock graphs are... um... less than useful.

In the glucose level sparklines he showed the "normal zone" with a grey bar, which does provide the scale. I don't see how the same thing could be accomplished for a variable that doesn't have a normal zone - for example, anything that exhibits growth behavior (stocks, weblog hits, etc).