Include dependency

This is the recommended way if you use Android Studio / Gradle. You can require the library from the Maven Central Repository.
Add that line to your build.gradle file under your app directory into the dependencies block:

compile 'com.jjoe64:graphview:4.2.1'

Now you can proceed with creating a simple line graph

Alternative way

Clone the github repository and link your project with the GraphView library project. Then you have full control over the sources and can modify or debug it directly.