To accurately predict the moon phase on December 15, I would need to see the graph you mentioned for the moon phases. However, without the visual data, I can provide general guidelines about the moon's cycle.
The lunar cycle typically lasts about 29.5 days, going through the phases: new moon, waxing crescent, first quarter, waxing gibbous, full moon, waning gibbous, third quarter, and waning crescent, before returning to a new moon.
If you know the moon phase on a specific date leading up to December 15, you can calculate the predicted phase based on the number of days between that date and December 15.
If you provide the moon phase from earlier in December, I can help make a more specific prediction!