Scrum overestimation and replanning

Question

We are in the middle of our first Sprint and something dawn on us: we over estimated!

We had planned 114 ideal hours for this 2 weeks iteration and at the end of the first week we finished the whole Sprint. What do we do now? The "book" says we should, and we will, get the next high priority items from the backlog. Though, how do we add them to the burn down chart? Do we re-write it to account for those stories as if they were there from the begining? Or simply add their estimates to the y axis in the day we start working on them (showing a 90o angle jump)?

Any feedback is welcome!