Command to show graphs for number of new/active volunteers

[seeroflights]

Would be interesting to be able to run graphs for number of new or active volunteers for a given time period (I guess visually+usage kind of similar to buttercup's /rate?).

For example, the school year (for northern hemisphere at least) seems to have an impact on both numbers; it'd be interesting to see how that compares to previous years, or to other bumps like end-of-year holidays.

It would also be interesting to see whether the number of active volunteers we have drops off or rises coincidentally with anything.

Currently for new volunteers the best way to do it is essentially "pure vibes", and for active volunteers I've been making do with running /rate users: all for certain timeframes (obviously this is not the most accurate solution, since Transcribers Georg, who does 100/24 every day (gestures at self) is an outlier who should not be counted.

[TimJentzsch]

I think it would be better to add this to Buttercup, I don't think we need to keep this information secret.

This also has the advantage that we don't need to add plotting dependencies to Blossom and honestly the slash command system in Discord is better than in Slack IMO.

[seeroflights]

Sure, I'm cool with that! I'll edit the title/description a bit and then try transferring this