Editing Graphics development with vplot (section)

==External variables and programming technique==
He who assumes that any external variables he can get his hands on
he is free to change will get what he deserves: BUGS. In general, even
if you THINK you know what a global variable used by the vplot
device-independent routines does, if it isn't specifically documented
here that you can use it, LEAVE YOUR HANDS OFF OF IT! (Well, occasionally
you may have good reason to tweak a few variables; vppen does this. In
general, though, be careful!) If there were some way to limit globals
to certain groups of routines in C, I would have done it for many
variables; but there isn't, so I didn't.

In particular, if you need to keep track of the device's state, don't
rely on variables associated with the device-independent routines to
do it for you. They may not match with the actual state of the device.
Do it yourself, with your own set of device-dependent variables! Don't
assume that you know exactly how the device-independent code works!