In the attached Excel file, the first sheet (membership) records the average number of members per month over the period 1920-35. The second and third tables (atten_perccent, attend_nbr) record the monthly average percentage of attendance and actual number of attendees, respectively, over the same period. The last table records the total number of meetings per month. Empty cells reflect missing information. 

Original data come from the Rotary International Archives, more particularly: 

• 1920-28: Shanghai, vol. 4. File no. AC0033-B12-F01-pages085-142 (p.15-58)
• 1928-34: Shanghai vol. 6. File no. AC0033-B12-F03-pages095-138 (p.18-45)
• 1934-35: Shanghai, vol. 7AC0033-B12-F04-pages071-128 (p.16-21)
• 1935, July-Sept: Shanghai, vol. 7. File no AC0033-B12-F04-pages129-176 (p.31-32)

I also attach the script and four text files I used for analyzing time series using R Studio (see the "Graphs" section).