What expression can be used to graph an equation with digits using f(x) (each digit of the number corresponds to a value of x where carrying over is okey and

What expression can be used to graph an equation with digits using f(x) (each digit of the number corresponds to a value of x where carrying over is okey and when the value goes above 9 it goes back to 0 and carries the number over)? For instance: f(x)=3x, and the first 10 digits of the awnser are: 3702591470