differential graphs: make neutral slightly gray.

[smoofra]

If you plot a differential flame graph against a white background it can look really weird if there's a lot of neutral frames, because they'll be totally white too. This will make them slightly gray instead.

[brendangregg]

Makes sense, but as a performance tool some people would complain if I included a loop like this. I'd do it without a loop.

[brendangregg]

Closing for now (see my previous comment!).