Overlaying prior curves to multiple posteriors within the same plot

I’ve got a categorical model for 4 response classes, with varying intercepts for the 3 logistic sub-regressions. I’d like to plot the posteriors of the three group-level SDs, either as an mcmc_areas() or as mcmc_hist(), with the exponential(2) prior curve on top of each posterior for reference. Is this possible? The only thing I’ve been able to manage is drawing one single exponential(2) curve on top of the entire plot (see screenshot) rather than one scaled exponential curve for each posterior:

I imagine there should be a simple and easy solution, but I haven’t been able to find it yet.

Suggestions?

I still haven’t found a simple solution, but I managed to jury-rig it by constructing a data frame with the exponential(2) prior densities for relevant parameter values, then adding three geom_line() calls to add these density curves on top of the mcmc_areas(). Given that there are three PDFs in one plot, these constructed exponential densities had to all be divided by 3 and assigned y intercepts equal to 1, 2, and 3.

The result won’t win beauty contests (see screenshot), but it’s better than no overlay at all.

I’m still curious to know if a prettier and more convenient approach exists.