Piggies are all different in what they eat, I have discovered! Some of mine liked banana, the current pair won't touch it. In fact they're not fussed about any fruit, including apple, which was always a favourite with previous pigs.
Limit lettuce and spinach because of oxalates --> kidney stones; they LOVE it though so you can give it as an occasional treat, and only a little bit.
Parsley is an excellent source of vitamin C for them.
Tomatoes, apples and other acidic foods - cut up into relatively small pieces, not for choking but because it can cause sores on the edges of their mouths.
My 2 pigs get daily (for both of them): one large tomato, cut into 8; 2" of carrot, sliced lengthways (good for keeping their teeth down); one large stick of celery, including leaves, chopped into ~2" lengths; 2" cucumber, quartered in lengths; handful of flat-leaf parsley from the garden; plus they are outside so have grass to crop from the lawn and their pellet food. This pair don't like broccoli 
but they will eat cauliflower leaves when I have them.
Other pigs I have had have loved apple, strawberry, grapes, banana, broccoli, kale - but not this pair (and they're not even related, just Australian)!