try the default bitmap type

vignettes/xfun.Rmd

@@ -22,6 +22,7 @@ output:
 
 ```{r setup, include=FALSE}
 library(xfun)
+options(litedown.bitmapType = .Options$bitmapType)
 ```
 
 After writing about 20 R packages, I found I had accumulated several utility functions that I used across different packages, so I decided to extract them into a separate package. Previously I had been using the evil triple-colon `:::` to access these internal utility functions. Now with **xfun**, these functions have been exported, and more importantly, documented. It should be better to use them under the sun instead of in the dark.