Answer: Hello!
The graph of a function is linear is the function in it is a linear function.
A linear function is a function of the form f(x) = ax + b, where a is the slope and b is the x-intercept, both real numbers and constants.
In this case f(x) = y, and in your graph you put the pair (x,y), where the relation between y and x is a linear relation, and its represented by a linear graph.