I'm building something similar to the reputation graph / summary on the SO user page.
The answer explains that the /users/{ids}/reputation API method gives a sum of reputation for the period. So querying for the start/end of 14-Apr-2011 gives me the +5 rep I see in the screenshot.
The screenshot graph runs from 14-Apr-2011 to 16-Mar-2011. To build something similar I think I'll have to hit a URL with the date boundaries for each day in the range. Is this right, or is there an easier way?