if (selectionX >= 0 && selectionY >= 0)
{
    Debug.DrawLine(Vector3.forward * selectionY + Vector3.right * selectionX,
        Vector3.forward * (selectionY + 1) + Vector3.right * (selectionX + 1));
    Debug.DrawLine(Vector3.forward * selectionY + Vector3.right * (selectionX + 1),
        Vector3.forward * (selectionY + 1) + Vector3.right * selectionX);
}