Re: Need Help regarding Reference line in area chart.
I had very similar situation in the past and I was struggling to get the reference line for changing numbers. I mean if the reference line represents a static value across your Dimensions then it is very simple. You can add the same by going to Presentation Tab > Reference Lines. However, in your case you need the reference lines ([HEAD COUNT]) to change based on the Time Dimension.
I am attaching a solution which I have used in the past. It is not 100% solution you are looking for but the it deliversthe results. I have used two charts and overlayed on each other. And used the transperancy and other visual properties to depict as one chart. Most important thing to remember is to lock the position of the chart. You can do the same by accessing Chart Properties > Layout > Allow Move/Size (Uncheck the option).
Also, I am not sure the reason why you have used variable in the Aggr() function. My understanding is that you have used it to capture the Dimension Group Field Name. If that's the reason then you don't need a variable (ofcourse, which is additional overhead for calculation)... you can rather use the Dimension Group name within double quotes - "PERIOD" in your example.