Add ability to set yscale to go to zero

In some cases one wants to see what the changes are on an absolute scale. For example here:

http://amras1.github.io/astropy-benchmarks/#

I'd like to see it on a scale that goes to zero. Maybe an option could be added to go from 0 to max * 1.2 or something similar?
This should also apply when the units are bytes, so should probably go right after the determination of min/max above in its own `if` block.