Example URL:

http://svnview.digium.com/svn/libss7/team/mattf/libss7-ss7cluster/isup.c?view=diff&rev=288&r1=287&r2=288

If you edit that URL and replace 'svnview.digium.com' with 'code.asterisk.org', you'll see what Fisheye displays in ViewVC compatibility mode. It properly moves to revision 288 of isup.c, but it apparently ignores the 'r1' and 'r2' parameters in the URL, instead it displays the entire content of the file as of revision 288. Using the 'from revision' drop-down list in the display to select revision 287 results in the proper diff being displayed.