I have a dashboard for a headcount. 

As you can see there are 2 lines lines. One is the total running count of people still working for me and the other one indicates people who have left. 

Is there a way we can combine the 2 lines and depending on the data, it either keeps a running total (if people keep joining me) or reduces from that total (if people leave)?

Is there a setting for this or is something possible via query?