I've been digging around but have not found a plot showing a typical peak pressure/velocity relationship for modern cartridges. Tried Google and searched here with no luck, checked NRA's Handloading, nada.

I'm going to guess that with pressure as the independent variable on the X axis and velocity on the Y axis, the curve is not quite linear at the lower end, with increasingly negative curvature at the upper end, and a clear peak and falloff at the high end. I'd sure like to see such a plot, if only to scratch this itch. Doesn't really matter what cartridge, though I guess one of the standard rimless bottlenecks would be nice.

Does anybody have a link to a plot, or can post a pic?

thanx

Look here
http://www.shootingsoftware.com/pressure.htm
Maybe this is a partial answer. The burn rate is also a function of the graphs shape. Hope this helps.

As most people measure velocity it is usually
displayed as the x axis. Such plots are common place. Nowhere are they "linear".
Good Luck!