Filling between two lines / Slope before stacked area

The trick to getting a "vertical" boundary between stacked areas is to make sure the horizontal axis is a date axis, then have two entries for the same date. Because you are using a multi-level category axis, your current horizontal axis cannot be a date axis. Assuming I understand what you want, here's what I did:

1) Knowing I need to get rid of the multi-level category axis so I can use a date axis, find the series that defines the horizontal axis, and change it:
1a) Bring up the select data dialog.
1b) Select each data series in the "Legend Entries (Series)" window until I find that the Establishment series and the Staff in Post series have the horizontal axis definition.
1c) With either of these series selected, click on the Edit button for the horizontal category axis labels and change so that it is only referencing to row 3 (Z3:AC3)
1d) Select the horizontal axis -> format axis -> check the "date axis" radio button to be sure it is using a date axis.
1e) Other axis formats as necessary (such as a number format to show the dates if Excel does not do it automatically).
2) I need two entries for April in order to get a vertical boundary, so I select column S and insert a column.
3) Select R3:R11 -> copy -> select the new S3 -> paste.
4) In R8, enter 0.
5) Any other formats as needed.

If I've understood, then that should do what you request for this specific example. Did I understand? Does that work for you? What steps do you have trouble with?