By definition, the graph of a function in the cartesian coordinate plane is the plot of the curve that join all the points that satisfy the equation of the function.
In the cartesian coordinate plane, each point is represented by a pair of coordinates (y, x). y is the value of the function f(x), and x is any input value of the function.
In this case, the function is given as f(m), which means that the input value is represented by m.
Hence, the graph is the plot of y = f(m).
The choice of the letter used for the variable is completely arbitrary and does not change either the math or the graph. It could have been said that the function is f(x) in which case the graph would be y = f(x). Nothing changes but the name that you use.
