Does TortoiseSVN Revision Graph draw a line from Branch back to the Trunk when I finish a "Merge"?
Taking a branch is a copy operation (which is very cheap in svn) and it looks obvious in the logs that is what happened, hence the line. It's obvious (to svn/tortioiseSVN) because you can't make a branch without taking every single artefact from the trunk at the revision you chose. That's why a line is always shown - making a branch is guaranteed to take all your files from the trunk into your branch. Merging back in is different, you can selectively merge in whatever you want.
Merging back in is not a copy but a commit of your diffs from the HEAD (current latest trunk) to your own working copy (your branch). To SVN, it looks like someone took the latest trunk and changed a load of files at once and committed them in.
IMO, a line going back to trunk would be misleading because you could merge and choose the trunks version over your own for every single file - the line would imply you had brought all your changes in but there is actually no guarantee you brought any in.