How do I add a simple connecting line to my XY graph?

This is not automatic in Prism, because users so frequently add a fitted curve to their graph rather than just "connecting the dots", or they don't use a line at all.

To add a connecting line, double-click on one of the symbols on the graph to open the "Format Symbols and Lines" dialog. Verify in the "Data set" box that you are working with the correct data set, or check the "All" check box to make your settings effective for all your data sets. Now check the "Connecting line" check box. Choose the line's color, style (straight vs. staircase), thickness, and pattern (solid, dotted, dashed, etc). Don't confuse the connecting line, which always goes from point to point, with a best-fit curve or a line created by regression. If you have a curve through the data, Prism sees that curve as a separate data set. The data set for the experimental values themselves should be formatted to show symbols but no connecting line, while the "data set" for the curve should be formatted to show the line but no symbols.