1 teaspoon red-wine vinegar
1/4 teaspoon agave
1 tablespoon olive oil
Pinch of salt and pepper
1 1/2 cups halved cherry tomatoes
1 small cucumber, peeled, halved lengthwise, seeded, and cut crosswise into 1/4-inch slices (about 1 1/2 cups)
1/2 cup red onion, thinly sliced
1/4 cup shredded fresh basil leaves