Make Pita Chips: Preheat oven to 400°F. Brush the pitas with olive oil and cut into 6 wedges. Place them in a single on the baking sheet, sprinkle with salt and bake for 10~13 minutes or until they are crispy and crunchy. Turn the sheet halfway through baking for even cooking. Let cool and keep ready. These can be stored in an airtight container for later use as well.
Make Tzatziki sauce: In a bowl, combine Greek yogurt, dill, lemon juice, garlic, salt and pepper. Mix well.
To make Greek Nachos: Arrange pita chips on serving plate, top with chickpeas, cucumber, tomato, red onion and black olives.
Spoon the tzatziki sauce over the pita chips and sprinkle with feta cheese (if using) and serve immediately.