When you configure a time period for the Rank Tracker module that's more than 90 days in the past, you'll notice that only one data point is shown per month. 

This is because we archive daily ranking data that's more than 90 days old, in an effort to maintain a reasonable level of server utilization and keep our prices affordable. 

We do have plans to upgrade our server infrastructure down the road, and we may be able to increase the daily rankings data retention period to 180 or 365 days at that time.