The problem is that you are zooming in so much that the points used to draw the line are so far away from the area being zoomed into that the line shifts away from where it should be. Even though there is data visible, the curve fit line moves because only the end points of the fit are used in drawing the curve fit line (it is not drawn from point to point). One workaround is to include some points within (or just outside) the limits that you wish to view. This way the data points aren’t spread so far apart and you can zoom in further on the data.
KaleidaGraph is a thoughtfully designed graphing and data analysis application for research scientists, as well as for those in business and engineering fields. It produces publication-quality graphs, and easily converts the most complex data into a functional display.