suggests, without retracing the work (see the FAQ: How to do a minimal reproducible example reprex for beginners](FAQ: How to do a minimal reproducible example ( reprex ) for beginners); it should be cut and paste), that f(V) will also be unique, but it does not guarantee that the return values of f will necessarily appear continuous along both axes. Re plot as a line graph and see if there are not discrete breaks on x around 0.18, 0.44, 0.46, etc. To get the y axis to look more continuous, change the aspect to compress the $y scale.