Not clear whether x is the domain of the f function, or if you're considering another implicit variable, such as t for instance representing the time line, therefore I'm assuming the 2nd case.
You can use the y>x constraint to create another curve ( e.g: z ) which contains the same value of f. You will now plot z(x,y) instead of f(x,y), but setting the range of the plot function as the size of the z funcion, such as something like that ( not tested ) :