Spurred by Godrulz question regarding tithing, here is the "voluntary donation" graph for one congregation (https://anonfiles.com/cdn/1310042378588.pdf). I know this is statistically not precise enough as the data set is small but it shows the trend of Worldwide Work donations.
For those that have no insight into the different funds:
Worldwide Work = Sent to the branch offices for various expenses (mostly given as a donation for Watchtower literature)
Kingdom Hall Fund = Sent to the branch offices for building and repairing Kingdom Halls
Local Congregation = Kept and used for the local expenses. Local expenses include a 'tithe' from the WTBTS for the Circuit Overseer Fund which pays for CO's gas and car lease (~$70/month, other expenses made by the CO are charged separately whenever the CO comes) and the Kingdom Hall Assistance fund (~$55/month) which is a type of insurance fund for emergency repairs (which also get charged back to the congregation afterwards). It is recommended that any leftovers be sent to the branch in a 'non-interest bearing fund' which apparently is set up as a savings account but is very hard to get stuff out (they won't allow large sums to be taken out without prior authorization).
Special funds = Set up for large projects such as renovations, new builds and expansions for that congregation. We needed to raise about $250,000 so I guess we'll need an interest-bearing loan (2.5% interest per year) for a revolving 10 years + fee's if you can't pay the full amount for the month + fee's if you can't make the payment within the 10 years from the WTBTS. The WTBTS also disallowed use of our 'savings' account (>$10,000) with them for this project.
I made a trend line only for the Worldwide Work, the rest of the funds are trending fairly flat. You could argue this is because our special funds or the economy but it felt it was already going in a downwards trend over the years (don't have the accurate data though) and the economy has been looking up recently.