When people brag about 50% retention, it's trial to conversions.. When they talk about rebill %'s, it's first months sales.
Just because I have 4 guys join, 3 cancel, that appears to be a 25% rebill rate, but if that one guy rebills 9 months.. my retention rate isn't 25%. Your graph doesn't factor in the compounding rebills, which are not a steady 50% for any company.
Time and rebill cycles are a factor in the retention rates of sites. And with time, you should convert people to money values.
If you want to use 50% retention numbers, you would use sales+rebills vs. the cancels after the day ends. If you can keep your cancels 50% or less of your sales+rebills, you are growing.
But rebills are king
And we aren't anything like the mainstream business...