improve display when the zooming section have only one point.

experiment Q330 5244. When zooming section have only one point, min_y and max_y will be the same which make the display area collapsed.

added PriorityMedium label

assigned to @ldam

added Type: Enhancement label

mentioned in merge request !150 (merged)

mentioned in commit 7341e59b

closed with merge request !150 (merged)