Calculate HR trend graph

Hi All,

I'm having a bit of an issue working out the measure. 

So all I am trying to do is put together a trend graph for how many employees we have at each year, month & Quarter range (depend on selection but have the ability to do so), but keeping the ability to cut by slicers for info such as Departments etc. 

I have the following info in column: 

Start Date, 
End Date, 
Current or Historic 
Department

Been struggling on this for ages, would really appreciate any guidance, I've seen some simliar threads but couldn't quite apply on to my situation. 

Any help will be great  

Cheers
Simon