If you want to do modifications on the codebase of GraphView, it is meaningful to include GraphView as submodule into your application. If you can use the default implementation of GraphView it is recommended to include GraphView via gradle or as .jar file. See here for instructions about that.
Here are the steps, how to link the GraphView module into your Android project, to have to opportunity to do modifications on the codebase of GraphView.
Step #1: Clone or download git repository
$ git clone https://github.com/jjoe64/GraphView.git
Step #2: Import GraphView as Module
In the Android Studio select the menu item "File > Import Module..." and select the folder of GraphView that you have clones before.
Step #3: Add dependency
Open the build.gradle file of your app-project submodule, this is typically located at [project-folder]/app/build.gradle.
Add this line to the dependencies block: