Making my own graphics device

I want to make my own graphics device am thus looking for
documentation about graphics devices.

The only thing I've found so far is these directions for
making graphics devices with the RGraphicsDevice package.
http://www.omegahat.org/RGraphicsDevice/

Could someone point me to any other resources? Or just
some documentation about how to edit base R? If I don't
get anything, I'm just going to stare at the grDevices
section of the R source code (src/library/grDevices/src)
until I figure out how it works.

In case you're curious, I want to make a graphics device
that saves the graph in Hewlett-Packard Graphics Language.
https://en.wikipedia.org/wiki/HPGL

Thanks

Tom