I wanted to export just the legend into another program, but Prism always includes the entire graph when it copies to the clipboard or exports to a tiff or jpg file.

You can't do it from the graph, since Prism always includes the entire graph. But here is a way to work around that limitation. 

1. Create a new empty page layout.
2. Delete the placeholders (click on them, then press Del).
3. Copy the legend from your graph, and paste onto the empty layout.
4. Copy and paste (or export) from that layout to your other program.