With a serrated knife cut artichoke stems flush with bottoms. Cut off and discard top inch from artichokes. In a saucepan cover artichokes with salted water and bring to a boil. Simmer artichokes, covered, until just tender, 20 to 25 minutes. While artichokes are cooking, coarsely chop bacon and in a skillet cook over moderate heat, stirring, until golden. With a slotted spoon transfer bacon to paper towels. Halve and seed tomatoes and cut into 1/4-inch dice. Finely chop shallot and capers. In a bowl combine lemon juice and salt and pepper to taste. Whisk in oil in a slow stream until emulsified and add bacon, tomatoes, shallot, and capers. Drain artichokes upside down on a rack and arrange each on a plate. Separating leaves slightly, drizzle vinaigrette between leaves and onto plates.