The full range of Nikkor primes

DigVis wrote:

Wasting half an hour, I wrote a quick script that parsed Roland Vink's excellent site

http://www.photosynthesis.co.nz/nikon/lenses.html

and produced the following plot of where all currently sold Nikkor prime lenses are placed in the focal length/maximum aperture design space.

Some interesting patterns appear. For example, there seem to be an obvious gap at around 18/2. With the 18/2.8D being such a poor seller (7641 total sales since 1994 based on the highest reported serial numbers), I can certainly understand Nikon's hesitancy to target this design point. What were the reasons for the apparently abyssal sales of the 18/2.8D? Is there a viable market case for a new fast 18 mm prime?