To accurately predict the moon phase on December 15, I would need to see your graph or have some information about the moon phases leading up to that date. If you can provide details or the pattern from the graph, I can help you determine the likely moon phase.
Typically, the moon cycle is about 29.5 days, and if you know the current phase on a specific date, you can calculate the phase on December 15 by counting the days. Please share that information!