Error: polygon edge not found when using bayesplot

Hi there!

Thanks for maintaining bayesplot!

I've got a bit of a funny issue that I was wondering if you have encountered with bayesplot. I find that sometimes, but not always, GitHub actions fails building a vignette when calling a function from bayesplot, giving the error:

Quitting from lines 469-474 (get_started.Rmd) 
  Error: processing vignette 'get_started.Rmd' failed with diagnostics:
  polygon edge not found
  --- failed re-building ‘get_started.Rmd’

(GH action line here: https://github.com/greta-dev/greta/runs/5233369273?check_suite_focus=true#step:11:245)

This is occurring at this line of the vignette:

https://github.com/greta-dev/greta/blob/master/vignettes/get_started.Rmd#L469-L473

Having a look around, it initially appeared that there was an issue with fonts: https://community.rstudio.com/t/ggplot2-polygon-edge-not-found/116475/4 - however, taking a look at your code it doesn't seem that there are custom fonts loaded, so I'm not sure why this is happening.

In any case, please do feel free to close this issue, I just thought in case you've come across this before, if you have any tips on how to resolve this!

Cheers!

Start with vignettes/get_started.Rmd around lines 469-474 and inspect the GitHub Actions run linked in the report. Reproduce the intermittent vignette build failure and determine whether it depends on the plotting call or the build environment. Done means the vignette builds reliably in the affected workflow, with the cause and resolution documented.