Check any lines that you created using the Line/Arrow tool. One or more of them may actually be an arrow instead of a line. If you double-click it and the Edit Arrowhead dialog appears, it is an arrow. To convert it to a line, click the line segment that you created in the plot, click and hold the mouse down on the Line/Arrow tool, and select the first item in the pop-up menu (the normal line segment).
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.