I'm poking about in the usual places, but not having any luck, does anyone have any good introductory information for HP's AGL, "A Graphics Language"?
AGL was an extension to BASIC and FORTRAN, which was implemented on several HP systems. It was used to describe what you wanted drawn, and the BASIC/FORTRAN system would generate HP-GL to drive a plotter.
The most accessible manuals seem to be here. The HP-86/87 Plotter ROM Manual, describes the changes to the HP-86 BASIC when using a plotter. These don't seem to be very extensive, suggesting that you also need to look at an HP-86 BASIC manual to learn how the graphics commands worked. However, that manual doesn't seem to be on that site, although the HP-85 manual is.