I don't understand those charts at all. Some charts show growth (right side of line is higher than left side of line) but the growth figure is negative. Some charts show decline, but the growth figure is highly positive. I have no idea what those charts are supposed to be illustrating - seems to be random noise.
I believe it's all constant growth, spike aren't indicating the growth, just a spike in growth. If that makes any sense. So if it's flat for example it's 100/day, if it's going up it's probably a spike over an average day, say 200/day for that day. I think the give away is that people don't seem to unfollow on twitter.
The charts are showing growth over time, as in delta change on any given day from the previous day.
Imagine you had a normal chart where each point is the absolute number of followers, but you take the derivative of that function (or use the deltas between days instead).
The data is only being displayed for the past 30 days. Keep in mind, just because it's flat doesn't mean it's 0. It could be constant at a higher number of growth. My apologies for not showing that effectively.
