finish line2poly function and vignette

[mpadge]

Has to be checked to ensure everything works okay, and the vignette probably doesn't need all of the small maps which junk up the /vignette directory (or maybe just make these on the fly using pre-stored data). cc @richardbeare

[richardbeare]

Would suggest keeping the small maps somewhere as test cases. I wasn't sure where to do that. They definitely don't look good in the vignette.