We have a 28" USCutter vinyl cutter. It speaks HPGL.
On Windows, we use the VinylMaster software.
There are two methods available to InkScape users.
Use Inkscape to create a vector image or convert a bitmap into a path
Save as *.hpgl
From the terminal, cat file.hpgl > /dev/usb/lp0
Converting bitmap to vector
Select the object, then choose Path > trace bitmap