Scaling that preserves the ratio between any two values

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Is there a function $f$ such that for all $x$ (on a bounded domain):

  1. $f(x) \in [a,b]$
  2. $r \times f(x) = f(rx)$

If it makes things easier, we can assume that $\forall x: x \geq 0$. Though a more general solution would be nice.

P.S. I figure that it may be impossible to find such a scaling under some circumstances. For example, a scaling that maps $x_{max}$ to $a$ and $x_{min}$ to $b$ obviously needs $\frac{a}{b}=\frac{x_{max}}{x_{min}}$