URLs in Legends (and Notes) are clickable in SVG, eg see the bottom of this diagram.
URLs of whole nodes are clickable in SVG and PNG.
But URLs in Legends in PNG are not clickable, eg see this form (top left node is clickable, link in legend on the bottom is not).
It would be nice to compute URL areas in Legend & Note and add them to the imagemap.
A related minor request: make the URLs underlined, rather than having to use a <u> tag around the link