Approach for calculating past and predicted charging station occupancy

I have an application that revolves around booking of electric vehicle charging stations. The data model consists of a ChargingStation entity and a ChargingSite entity which both are specializations of a Location entity. A site can have multiple stations. There is also a ChargingSession entity that is used to book and record time slots when the chargers are in use. Now I want to create a time series graph that show occupancy % per site, based on both past sessions and planned future sessions. How would you approach this in terms of entities and microflows?   I have been considering creating an additional time series entity that records the occupancy level (0 or 100) per minute and charger to make the graph generation quick. From this I can summarize occupancy per site based on all chargers in the site.  I think this would work fine for all sessions that has happened, but any future sessions can (and will) change, so I'm not sure how to handle this.   Do you have any tips or best practices when it comes to time series data? Thankful for any tips! 
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