Ok ! I see, I suggest you to look TestEm1 example. In particular TrackingAction Class and Run Class.
The graph you presented is from the G4Step class. i.e. G4ThreeVector trkPos = step->GetTrack()->GetPosition(); say Z = 1mm, you are scoring under process hits in steppingAction ==> depth = trkPos.getZ();
You can also read comments here “Geant4 and SRIM radon-222 range agreement - #9 by hellinger523”