Check the Curve Fit Options dialog (Curve Fit menu). The Curve Fit Points field could be set to 2. This would cause KaleidaGraph to calculate two points for the fit and draw a line between them. This field is normally set to 100.
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.