Hi gaocaiping, in this context it means that they neither gone, nor gone down, so they did not rise or fall, but basically remained around the same level.

"To track" means to follow the trail of an animal, in order to see where it is going. If you have data that makes a track, that can be followed, then you could track it. For example, http://centreforaviation.com/images/stories/2014/Oct/18/Cargo12.JPG The data is like a trail, when shown as a graph. In your sentence the verb "tracked" is used to say which direction the trail has been going. As Ceejay said, it hasn't gone up or down much. So to the side, or sideways.