Workforce development graph based on contract dates

Hi FlorisMK ,

In order to deal with the headcount issue using dax, the best practice is to use the following tables which you have specified in your message:

• The employee table with start and end dates
• Calendar table 

and then set these tables as disconnected tables and write a measure like below:

Headcount =
SUMX (
    EmployeeFact,
    IF (
        EmployeeFact[Start] <= [SelectedDate]
            && EmployeeFact[End] >= [SelectedDate],
        1,
        BLANK ()
    )
)

For the details of how to set up the data model, please refer to the article below:
Dynamic Headcount Analysis using Dax - Microsoft Fabric Community

Best regards,