Charts for total pioneers, congregations, and hours per publisher would also be interesting.
You can make a case for the total number of congregations being the most enlightening figure over the long term:
1. It is in many ways the most reliable figure. Other figures can be distorted at various levels: publishers turn in fake hours, secretaries fill in reports on behalf of people who fail to report, irregulars get added to the peak total, plus it has even been known for branches to inflate their publisher and memorial numbers. However the number of congregations is likely to be a very accurate because it is hard to fake on any level. There is no such thing as a whole phantom congregation submitting reports, and the number of congregations in branches could theoretically be independently checked.
2. Other statistics may be subject to fluctuations due to temporary movements. However a congregation takes a long time to build up, and the decision to split or merge congregations is only taken when longer term trends indicate a need. Plus whereas changes in the hour requirements for pioneers distorts the meaning of those totals, the commitment level not just in terms of publishers, but in the number of elders needed to perform key tasks in order to form a congregation is fairly stable over the long term and is a good indication of the health of organization as a whole.
3. It gives a good basis for comparison with other religious organizations of a similar type to Jehovah's Witnesses. Attendance at congregation meetings falls fairly consistently within a range of around 50 - 120 or so. Bigger congregations tend to be found where the organization is expanding and smaller congregations where it is contracting. Comparable religious groups such as Mormons, Adventists, Assemblies of God, Brethren and so on have a similar pattern in the size of their congregations.
The growth in JW congregations has been more modest but steadier over time than other measures. It would make an interesting graph.
Mormons claim to have over 11 million members worldwide compared to Jehovah's Witnesses at around 7 million. However if you compare the number of congregations each organization has it shows a different picture. Jehovah's Witnesses have 103,000 congregations: does anyone know the latest figure for how many Mormon congregations there are? Crucially is the number of Mormon congregations going up or down?