//Shifting data to the left

int[] y;

void setup() {
  size(800, 600);
  y = new int[width];
}

void draw() {
  background(204);
  // Read the array from the beginning to the end
  // i.e. shifting values to the left
  for (int i = 0; i < y.length - 1; i++)
    y[i] = y[i+1];
  // Add new values to the end
  y[y.length-1] = mouseY;
  // Display each pair of values as a line
  for (int i = 1; i < y.length; i++)
    line(i, y[i], i - 1, y[i-1]);
}