Essexboy:
You're quite right, and I think we're actually in agreement: in practice the commonality benefits of a single-type fleet will considerably outweigh the operating cost advantages of the -700 on lighter sectors. My purpose in posting the graph was just to illustrate that there *is* an operating cost difference between the two variants.
Contrary to what some other posters have suggested, I don't think it would make sense for Ryanair to get -700s: sure, they'd optimise load factors in some cases, but they can no longer swap aircraft around as they want or they'd end up with overbooked flights. The complexity of their operation would go up - and all in order to save a relatively small % of operating cost on thinner routes.
To my mind, the conclusion to be drawn from these lower-load-factor routes of Ryanair's is not that they should be buying 737-700s but that they perhaps shouldn't be flying the routes. There are going to be some routes that can't just profitably be operated with a 737-800. (There will be others where the load factor is lower than average but the yields are good enough to make things worthwhile).
I recall a few years ago someone from Cathay Pacific (anyone remember who?) described the secret of their success as "the creative misuse of aircraft". He was referring to - for example - flying 747s on the one-hour sector from Hong Kong to Taipei in between intercontinental flights. The plane wasn't really optimised for that, but it made money and improved asset utilisation. I think that some of Ryanair's routes may be like that on a smaller scale.
C.