Fill a large pot with lightly salted water and bring to a rolling boil over high heat. Once the water is boiling, stir in the spaghetti, and return to a boil. Cook the pasta uncovered, stirring occasionally, until the pasta has cooked through, but is still firm to the bite, about 8 minutes.
Drain well in a colander set in the sink.
Meanwhile, heat the olive oil in a large skillet over medium heat. Stir in the garlic, and cook for 1 minute to soften.
Add the Swiss chard. Cook and stir until the stems of the chard are tender. You can use some of the hot pasta water to help steam the chard in the covered pan.
Stir the hot spaghetti into the chard mixture along with the capers. Season to taste with salt and black pepper, and drizzle with lemon juice if desired.
