If Flores del Mundo wanted to track membership rates over the course of a year, I would recommend using a line graph. Line graphs easily allow the user to look for trends over time in a visually appealing manner. In addition, if the data were split between Northern and Eastern regions, I would suggest using a line graph with a line for each region. A bar graph could also be used, but it may be visually easier to compare membership rates between regions with the line graph.
If graphing membership based on category (employee/spouse/dependent), a bar graph would be useful. Because the data captures an entire year, it may be helpful to stack the bars rather than place them side-by-side for each month. This method allows the user to examine categorical data as a part of a whole and is more visually appealing.