When looking at a user page and at "Reputation", hoovering the mouse over some day, then the tool-tip reads something like "Friday, Nov 25, 2011 - 42 reputation", which means "+42 reputation", not "-42 reputation". Would "Friday, Nov 25, 2011: + 42 reputation" (or maybe ";", "#", or whatsoever) not be more easily readable? (This format is used at other SE sites, too, probably everywhere.)
It seems like negative rep changes are not displayed in these graphs, it'll say "This user has no recent positive reputation changes". (Find a question with a negative score on http://meta.stackoverflow.com and go to its asker's profile to see that.)
But still, you have a point because people don't immediately know that only positive rep is displayed You should make your proposal at https://meta.stackexchange.com/questions/112151/new-user-page-live, where all the feedback for the new profile is gathered. I like the colon : solution best, but if they don't see your point at all, you can still point them to Use dashes in page titles instead of hyphens to demonstrate that the status quo is just unbearable
Status completed, issue solved:
Yes, I can definitely see some confusion here, the format will use a : instead of a
-in-between the date and rep next build. – Nick Craver♦ 2011-11-26 10:49:13Z
at https://meta.stackexchange.com/q/113693/171937 (has been deleted as solved).