Displaying active persons

Before you can create the chart you need a table of values that show at least these two things:

1. A series of dates
2. A count of people related to a specific date

So, you might start by populating a column with a starting date and filling it down as needed.
Then in an adjacent column, use a formula to count how many people were in service for a given date.

If you make the above into an Excel Table, you can build a chart based on the table that will automatically update as new data are entered into the table.

Hopefully, that gives you some direction to get started.