Thank you. I assume also other see a straight black line. However, this is a zoom of a circle!
Of course, when you plot a circle and zoom in enough, what you will see finally is a straight line.
This tells us: If you see a straight line this does not tell you if the total object is straight or round. You just cannot tell either.
Exactly the same: When you look out of your window and the earth looks flat, you neither can tell it is flat nor it is round. You cannot draw any conclusions from the statement "the earth looks flat when I look out of my window".