To accurately predict the moon phase on December 15, I would typically need to see the graph you're referring to. However, if I can't see it, I can provide some general information:
Typically, the moon phases cycle approximately every 29.5 days. If you know the moon phase on a specific date close to December 15 (for example, the phase on December 1 or December 14), you can estimate the phase on December 15 by considering how many days have passed since that date.
If you have data indicating what the moon phase was on a recent date or a full cycle to refer back to, I can help more specifically. Please provide additional context or details about the graph.