I was interested in finding out how much reputation a user gained during a certain period of time and I couldn't find an easy way to do it.
I believe the reputation graph already provides the visual elements for this feature: on the left corner there's a label with the total amount of reputation (circled in red), which is kind of redudant since the top of this page (which got cropped from the image below) presents regular user info, which already includes the total amount of reputation.
In other words, it seems there are 2 labels in the same page that display the exact same information.
My suggestion is that the label circled in red be updated to reflect the amount of reputation gained during the period of time selected in the graph bar (red rectangle).
I think it's an interesting feature to have.