In a glass mixing or serving bowl, stir together vinegar and agave until smooth. Add corn kernels and onion and allow relish to stand while chicken and plums cook.

Adjust oven rack to upper middle position and heat broiler. Season tops of chicken with salt and pepper. Leave a little room at one end of the broiler pan for the four plum halves. Broil chicken for four minutes.

Brush the cut side of the plum halves with the olive oil. After chicken has cooked for four minutes, remove the pan from the oven, use tongs, not a fork, to turn the breasts over. Place plums, cut side up, on broiler pan and return pan to the oven. Cook three minutes, turn the plums and cook two more minutes. Remove the plums and chicken from the broiler. Allow chicken to stand while you dice the plums. Peel off any loose skin and use a fork and knife to cut the plums into smallish bite sized chunks. Mix plums with the corn and onions.

Serve chicken with relish on top or on the side.