Cut stems and thick center ribs from Swiss chard leaves. Discard center ribs and chop coarse stems and leaves separately.
In a 9-inch heavy skillet cook onion in oil over moderate heat, stirring, until softened. Add stems, 2 tablespoons water, raisins, and salt, to taste, and cook, covered, over moderately low heat until stems are softened, about 5 minutes. Add leaves, olives, and remaining 2 tablespoons water and cook, covered, over moderate heat until leaves are wilted, about 3 minutes. Remove lid and cook, stirring occasionally, until most of liquid is evaporated and leaves are tender.