How can I make the plotting area on a Prism graph transparent?

You may wish to do this so that the background from Prism or another program (such as PowerPoint) shows through. It is possible for Prism 3 and later versions. Choose "Format...Plotting area color" and then select whatever color is currently the background color for the page (usually white). Prism will then ask if you prefer to make the background clear. Answer "yes."

You can learn more about making PowerPoint slides from Prism graphs in these step-by-step example articles for Prism 3 and Prism 4.