Answer:
Relation between Moon and Tide Cycles
Tide Cycles as they relate to Moon Cycles
Correlation between Shifts in Moon Cycle and Shifts in Tides
Explanation:
Your graph here is a graph of a relation, as most graphs / all functions are. So, you should include what you are comparing (full moons and tide heights).
Your x-axis is already shown to be measured by days, but you might want to include a more direct explanation {if you think that is what is being asked of you here}
possibly: Day of Month
and your y-axis is already shown to be measured in tide height, but you could write:
Height of Tide
to make it more formal {that is probably not expected of you}