I'm unable to see the graph you're referring to, but I can help you predict the moon phase based on the average lunar cycle. The lunar cycle is approximately 29.5 days long, which means you can estimate the phase of the moon on December 15 by knowing the moon phase from a specific date.
If you know the moon phase on a previous date, you can calculate how many days have passed and determine what phase it will be on December 15. For example, if you know the moon was a new moon on December 1, then by December 15 (14 days later), it would be a first quarter moon.
If you have specific details about the graph or dates, please share them, and I'll do my best to help you predict the moon phase on December 15!