Units always matter, Florian.

From your added linear control line it's clear that you're talking about a unit of voltage, or multiples of volts.  My ears do not register volts; they register dB, so the vertical axis of the graph, for audio level, should be the log of what you have there.

That being the case, your straight line becomes a curve, getting less steep, or flatter, as x increases, and your curved line may well become straight.

- Andy