Show Follower Count spikes and dips more clearly

[ginatrapani]

Ever since we changed follower count history charts from line charts to bar charts, it's much more difficult to see unusual spikes or dips over time, especially if you have a constant upward/downward trend.

Let's find a way to show spikes more clearly, either by reverting to line charts completely, or overlaying a line that displays deviance on the bar charts.

Discussed this one with @waxpancake already so assigning to him.

[cwarden]

Maybe adding a smoothed line to the plot would be useful, e.g.

You can see when the count deviates from the regression curve.

Or we could plot differences between sequential follower counts, e.g.

This might not be intuitive to users, though. The big spike in the middle of this one is due to missing crawls, but that could be fixed by interpolating missing data points. The other spikes seem to correctly emphasize spikes in the graph above.