I agree that a line graph would be a good choice for this data, but I also think that a bar chart/histogram would be a good choice as well. It would show the change in membership volume well from month to month. Additionally, the Flores del Mundo data are not continuous data, they are cross-sectional, point-in-time assessments of the membership once per month. Because we are measuring membership at regular intervals, a bar chart could be a useful visualization of these data.
However, once we add the stratification by member type and region, it could be difficult to compare a bar for each group during every month. We could try a stacked bar chart, since we are only comparing two or three categories, but if a group is small, it could be difficult to see. This may be the point where I would switch to a line graph so that the reader can easily see the different groups on the same graph.