How to add a horizontal line to PivotChart in Access

I have an Access's PivotChart report showing, in essence, how much time (along 'Y' axis) different workers (along 'X' axis) are planned to work (e.g. in a given work day).  The "planned" time can be over or under the "available" (e.g. 8 hrs in a work day) time.  "Available" time is actually stored in a database field, not hard-coded.  

Now the question: How can I add a horizontal line to that chart at the level Y = "available" time, in order to visually spot the over- and under-utilized workers   (I couldn't find an answer at http://office.microsoft.com/en-us/assistance/CH062526661033.aspx .)

Thank you!