The two apparent curved lines is really a shaded section outlined by white lines so that it stands out better. It's a representation of how sensitive to light you likely are. The "fatter" it is, the more sensitive to light you likely are. It's based on scientific research.
The circle represents you. The circle's location on this light sensitivity 'graph' tells you what your sensitivity to light most likely is. At the time of your screenshot (provided all of your settings are accurate for you), f.lux was telling you that your sensitivity to light was on the decline, which makes sense because it was saying you wake up in 4 hours. The 'x-axis', as you described it, is where you are the least sensitive to light (in all likelihood, based on scientific research).
The dark orange line is bedtime mode.
The light orange line is sunset mode.
The blue line is daytime mode.
I don't have the Mac version (I have the beta version for Windows, which is very similar to the Mac version), but I think "12 sunlight hours" means that f.lux goes into daytime mode for 12 hours. So, I guess it's telling you that you could go out and get 12 hours of sunlight if you wanted to.
