Magnetic coordinates and lines drawing across the world map

I'm extending basemap to work with magnetic coordinates. Basically, latitude longitude are converted to/from magnetic coordinates during calls to Basemap.__call__(). However, I get results like this:

It seems that coastlines/polygons are not properly "broken" on the left/right plot edges. I suspect I have to change something in _readboundarydata() to fix this (since that's where the coastlines and polygons are initiated), and I have had a look at it, but I do not really see what I would need to change. I don't even see how this is handled in the normal (geographic coordinates) case. There are some references to the crossing of east/west datelines, are these "datelines" the left/right map boundaries in projections like the above? I don't really understand what this part of the code accomplishes.

Any tips?

Inizia riproducendo la mappa delle coordinate magnetiche mostrata in figure_1 e segui il modo in cui Basemap.call() trasforma le coordinate prima che _readboundarydata() crei coste e poligoni. Leggi la logica esistente per l'attraversamento della linea internazionale del cambio di data verso est/ovest per determinare come si relaziona ai limiti della proiezione. Il lavoro è completato quando coste e poligoni vengono suddivisi correttamente ai bordi sinistro e destro del grafico per le coordinate magnetiche.