The first image of BICEP2 visuals shows the "BICEP2 B-mode Signal", described as follows:
Gravitational waves from inflation generate a faint but distinctive twisting pattern in the polarization of the CMB, known as a "curl" or B-mode pattern. For the density fluctuations that generate most of the polarization of the CMB, this part of the primordial pattern is exactly zero. Shown here is the actual B-mode pattern observed with the BICEP2 telescope, with the line segments showing the polarization from different spots on the sky. The red and blue shading shows the degree of clockwise and anti-clockwise twisting of this B-mode pattern.
I think I understand the red and blue, but I don't get what the line segments mean. Another web page explains a similar visual as follows:
Summing over all incident waves [at a given point], the E-fields are roughly equal in all directions, but not quite. There will be one direction that has a slightly greater magnitude of E than the other directions (see figure to the left).
We can represent polarization as a line with length proportional to the excess magnitude in that direction and at an angle such that it is aligned with the direction of largest E.
Aside from the fact that the latter description refers to E-mode, seemingly treating it as isomorphic to B-mode polarization (I'm talking through my hat a bit here)... I would expect from my understanding of this description that the length of a line segment should correlate to (the "absolute value" of) the intensity of red or blue at that point. But in the first visual referred to above, the correlation doesn't really seem to hold.
(BTW, I'm assuming that each line segment represents information about the point at the center of the line segment, not about a point at one end, unlike a vector field visualization would. That's because these are not vectors, since they don't really have direction in a 360-degree sense, but rather they have 180-degree rotational symmetry. This is consistent with the fact that each line segment's center seems to be on a grid point. Pardon me if I'm belaboring the obvious, but it took me a while to realize this.)
So... what am I misunderstanding here? What aspect of the polarization do the line segments actually show?
To summarize the question a different way: what is the difference between what the length of the line segments represents, and what the intensity of the red/blue color represents?