A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Murray Peake
Date: 2020 Feb 7, 00:54 -0800
Looking through the code I see there is one magic number in the source which is 'span' - the number of minutes of LST on the x-axis, which accounts for the aspect ratio of the Mercator plot vs the interval of latitude so that the distance scale is the same in any direction - that is, not different north-south to east-west. If I remember, this is a standard calculation if you're preparing your own Mercator plotting sheet, but I've forgotten the details...
The point here was to allow the course and distance to be laid off on the plot when taking a running fix.
Good luck with Gnuplot - I'm interested to see what that is capable of.