App Rank Monitor lets you keep an eye on your iOS app's ranking on the App Store
using a widget.

App Rank widgets updates at set intervals to keep you updated on how your app's doing, rank history is then saved in-app at your disposal.

Steps:

1). Search for your app which is in the ranking from the search window.
2). Tap the Add (+) button to add the app.
3). Go to your Home Screen, get into editing mode.
4). Add the "App Rank" Widget
5). Tap and Hold the widget and press the "Edit" button to configure.