This probably exists, but a check in the app store as not revealed it: there are loads of battery monitor apps, but it would be helpful to have a battery monitor app with a graphical output. So between charges, the app would get the battery % left every 15 minutes, for example, and then simply plot % v time. We would then be able to get an idea as to what is increasing the battery drain by turning things off, e,g. Siri at present, and see visually whether that has made any difference.
Just an idea for some app enthusiast.