Ok now that you see it, i will answer your questions:
The graph most likely has a curved shape because the opportunity cost is not constant. If at point BG you decide to start to produce more butter and less guns (moving towards B1G0), you move along the curve and experience diminishing returns. This means that a large fall in production of guns gives a relatively small rise in production of butter. This is because to increase production of butter resources need to be diverted from gun production, however these resources aren't suited to the production of butter and so not much more butter is produced - resources can't be freely switched from one production to another. This is known as factor immobilisation.