One option could be to use stages. After the user task where the approve/reject decision is made, create two stages, one for the approval branch and one for the rejection branch. Have the rejection branch steps in the rejection branch stage and have the approval branch skip right through that stage and into its own stage. Then in your reports, you could report on stage and it would show in-progress instances that were either approved or rejected. 

Alongside that, you can rename your end events to Approved End and Rejected End. You could also report on end event name so completed instances will show either Approved or Rejected. Between the two solutions, you can group in-progress and completed instances by whether they were approved or rejected.