Group Progress Dashboard y-ranges

I'm running a computation on 400 threads. I think that they're all pretty well saturated. The Task Stream plot stopped working well for me, so I thought I'd take @ian-r-rose 's Group Progress graph for a spin. It looks great!

But! The question I want to answer here is "am I saturating all of my cores". I have 400 cores and when I hover over the filled graph I see that I'm using almost 400 cores. Woot. But why is the y-range so much taller? I'd expect that using 400/400 cores (or n / n cores) to look like it was full of color with no whitespace. Also, it'd be good to have a y-axis I think.

@ian-r-rose thoughts?