I'm not certain how you derived your data, but another way to look at these gamuts is with 3D plots using Colorthink or similar software.
I used Matlab with the image toolbox. I have some functions that let me easily explore profile characteristics. For instance:
abs_lab_image=ProfileConvert(ref_image, 'EUPPG 9500 II TC9.18 with 25Grays RGB.icm', 'f', 1, 'r', 3);
Will render an image in Lab using relative colorimetric intent (the 'f', 1) then convert it back to the actual colors printed, (the 'r', 3);
Matlab makes it easy to quantify color changes using a variety of scenarios.

The colors are well outside of Adobe RGB as shown below:
Bill, I downloaded the 4226_ProPhoto.jpg but it is tagged as sRGB. If I assign ProPhoto it looks really awful on my wide gamut monitor. Am I correct in ignoring the sRGB tag and assigning ProPhoto? When I do that I have to desaturate 30% or so for the colors to start to look reasonable.
For instance if I examine the pixel at X=46, Y=93 I get RGB values of 213,0,36 which is Lab (52.8,125.1, 56.8 ). This particular Pixel has the largest deltaE2000 of 14.1 distant from Adobe RGB (1998) of any pixel in the image.