Anecdata, but I have a calibrated screen with about 95% AdobeRGB coverage, and the dark boxes and lines are very clearly distinguishable. Is your screen calibrated? Is something off with your contrast setting? Maybe a very bright backlight ruins the contrast ratio at the darker end?
(I agree, however, that it's not a very attractive color scheme, and not a terribly informative graph anyway. A simple X-Y plot would be better.)
These screens are factory calibrated (supposedly, but they included a colorimeter report in the box) and they're all consistent several years later so I'm guessing so. Maybe its gamma? Or maybe you're on OSX? (IIRC apple devices use gamma 2.2 by default and Windows does 1.8? I think?) Our difference could totally be because of default gamma.
I'm on Linux, and my gamma is calibrated to 2.2. 1.8 sounds way too low, I think! sRGB is roughly a 2.2 gamma curve, and a lot of professional media is done for screens expected to have a gamma of 2.4.
(I agree, however, that it's not a very attractive color scheme, and not a terribly informative graph anyway. A simple X-Y plot would be better.)