What granularity do you want for your chart ? days, weeks, months ?
I think you need to transform your start/end date table into a table of months say, with one row for each employee that worked that month.
Then you can just count the rows.
Of course, getting that table is the trick. You might be able to use CROSSJOIN() to get a new table that had a cartesian product for a calendar table and your employee table. So you would have an employee start date and end date for every row in the calendar. You can then add a measure that checks to see if the calendar date for that row is between the start date and end date. 1 if it is, 0 if not. Then you can filter the new table for the start and end date you want to use and just count the 1's in each period.
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