GraphView is null with values inside

This might be too summed up but if you are confused feel free to ask.

I am currently trying to make a self health check program. Where it would ask how u are feeling using a slider and it would display it in a line graph. However everytime I try to run my graph, it fails. Giving me the following error:

   Process: com.example.self_help, PID: 27448
   java.lang.NullPointerException: Attempt to invoke virtual method 'com.jjoe64.graphview.GridLabelRenderer com.jjoe64.graphview.GraphView.getGridLabelRenderer()' on a null object reference
       at com.example.self_help.MainActivity$2.onClick(MainActivity.java:83)
       at android.view.View.performClick(View.java:7448)
       at com.google.android.material.button.MaterialButton.performClick(MaterialButton.java:1194)
       at android.view.View.performClickInternal(View.java:7425)
       at android.view.View.access$3600(View.java:810)
       at android.view.View$PerformClick.run(View.java:28305)
       at android.os.Handler.handleCallback(Handler.java:938)
       at android.os.Handler.dispatchMessage(Handler.java:99)
       at android.os.Looper.loop(Looper.java:223)
       at android.app.ActivityThread.main(ActivityThread.java:7656)
       at java.lang.reflect.Method.invoke(Native Method)
       at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:592)
       at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:947)```

>The line where the error is, is as follows: 

                 graphView.getGridLabelRenderer().setLabelFormatter(new DefaultLabelFormatter(){
@Override
public String formatLabel(double value, boolean isValueX){
if (isValueX){
     return sdf.format(new Date((long)value));
  } else {
       return super.formatLabel(value, isValueX);
                            }
                       }
                   });```