I can't see the curves or connecting lines in my Prism graph, but they appear after exporting the graph to another program.

You may be exporting the file at a resolution that exceeds your screen resolution. So when you view the file, the viewing program has to figure out how to fit so many pixels onto a small screen. Depending on the program, you can see distorted pictures or lines. Try printing the TIFF file. Printers can handle a whole lot more dots than can a computer screen; the printed copy will probably look fine.