  Analogue gravity is based on a mathematical identity between quantum field
theory in curved space-time and the propagation of perturbations in certain
condensed matter systems. But not every curved space-time can be simulated in
such a way, because one does not only need a condensed matter system that
generates the desired metric tensor, but that system then also has to obey its
own equations of motion. And specifying the metric tensor that one wishes to
realize usually overdetermines the underlying condensed matter system, such
that its equations of motion are in general not fulfilled, in which case the
desired metric does not have an analogue.
  Here, we show that the class of metrics that have an analogue is bigger than
what a first cursory consideration might suggest. This is due to the analogue
metric only being defined up to a choice of parametrization of the perturbation
in the underlying condensed matter system. In this way, the class of analogue
gravity models can be vastly expanded. In particular, we demonstrate how this
freedom of choice can be used to insert an intermediary conformal factor. Then,
as a corollary, we find that any metric conformal to a Painlev\'e--Gullstrand
type line element can, potentially, result as an analogue of a perturbation
propagating in a non-viscous, barotropic fluid.
