Battery Graph Widget tracks the battery level of your device and displays the graph with battery level. Furthermore, it calculates the estimate when the battery will be empty based on your past usage.
The graph and the estimates are displayed in the Today view of the Notification Center as well as in the app. The widget in the Today view is not displayed by default. You will have to add it with a few simple steps.
Both the graph in the app and in the Today view widget are customizable. You can select the color of the graph, translucency of the graph fill and turn on/off the grid lines.
In the widget, you can select one of the these time periods: 1/2 day, 1 day, 2 days or 3 days.
In the app, you can select one of the these time periods: 1 day, 1 week or 1 month. You can also use use the pinch gesture to set another period.
NOTE REGARDING COMPANY DEVICES AND BYOD
If you received your device from your company or you use your device as bring-your-own-device (BYOD) at your workplace, there is a higher probability that the battery graph will not be as accurate as you would expect. Company devices and BYOD's usually run software (such as VPN client), that requires additional resources on the device. This might negatively influence the graph accuracy.