By definition, extrapolation is when you estimate a value outside the data set and intrepolation is when you estimate a value inside the data set.
For the given data set, you know the revenue values from January, March, May, July and September.
Since the revenue for "August" is between the revenues from "July" and "September", which are parts of the data set, if you estimate its value from the data plot, then you would be making an interpolation.
