You can add any vegetables you have to hand. I find it's a good use of old veg and packets of herbs that are going limp in the bottom of the fridge but are not yet rotten enough to throw away.
Variations: Once, having no chives, I tried it with a tablespoon of finely chopped capers and a finely chopped shallot. It was OK as a standby but the chive version is definitely superior.