You’re just like almost there, that graph isn’t about lengths of days. It’s an arbitrary representation of what someone would consider a “normal” time for the sun to set and rise. The gradient in colors is because of the shape of the earth, the blocky lines are probably the cut off counties/states that follow different timezones (would need to verify source for that though). I’ve gotta split but hopefully someone can explain it better in the mean time if you have more questions.
Geez. that timezone separation is more fucked than I realized lol. I’ve lived in different time zones but always near the center of it and I didn’t know there was such awful cut offs that zigzag through states like that. Sucks that user stayed hostile though, not sure if they just want to argue about everything timezone related or just too embarrassed to gracefully exit.