Count the movement (add remove) of hosts

If I add 1 host and remove another host in a month, the stats will be the same and the delta zero but we had movement. Thats what im trying to track. This is what i have so far: | timechart span=1m dc(Host_Name) as Count_Of_Hosts | streamstats window=2 last(Count_Of_Hosts) AS Last, first(Count_Of_Hosts) AS First | eval Delta=Last-First