In the graph _ is a function of _. Please help

These are the multiples of 15 but what you are probably asking or what are the factors of 15

Thats where they was ported and the fields that noone really knew much about needed tended, so they put slaves out there do free work and huge profits and so on the north decided to hold its own and was more different way of survival did not have same climate conditions to afford protecting slaves from winter so expenses be diff from south