Perhaps this is well known information, but I have just stumbled across the Xfig application. (Search Xfig with your browser, it will be the Xfig.org)

This drawing, sketching application has the ability to export files in many formats including HP-GL. Quite useful for those using pen plotters.

The Application is available from the Linux package manager. Windoze users will need to find a way......

Cheers!

CalG