Is this what you mean? (attached)
I did a couple of things...
One was to add Metro Name to the detail on your Total EMP ...you only had it as an ATTR in the tooltip, and so was unable to send it as a dimension to the scatter plot (which is at Metro Name vizLoD)
and I changed the labeling on your Scatter to only show on highlight
Hopefully that does the trick, but post back if not...and I'll have another go!
MAIN METRO MAP 072016 - SR.twbx 429.3 KB
So it is the Filter that you want to act as a highlighter?...as of Tableau 10 (currently in Beta) there is now an option to do exactly this, however you have to choose if you want the drop-down to be a Highlighter or a Filter...as you can imagine you can't really set up a drop-down filter to act like one thing (filter) on one sheet and another (highlighter) on another...all in a single drop-down. So in your situation this would fix the scatter plot issue, but then your other sheets wouldn't filter!
I've come up with a solution using parameters. This should be fine here as 1) you only allow the user a single value select (and you can't do multi-select on parameters) 2) Parameters don't update dynamical as the data changes, even if you populate them from a field...but as you have a list of metropolitan areas it is unlikely to need, manually, updating very often.
So I've create this parameter and populated it from the [Metro Name] field. I then dual axis your scatter plot, and take the transparency right down on the first axis. One the second Axis, I use Shape mark-type and create the following field
[Selected Metro 1/0]
IIF([Metro Name]=[Metro Name Parameter],1,0)
and I bring this onto the shape tile...I then have 2 custom shapes (if you've not come across custom shapes, let me know)...one is a circle mark, and the other a blank shape (I just created this in paint.net, but any similar program will do). I then set the 1 to the circle, the 0 to the blank...turn up the transparency and dual/sync axis them (and hide the top header)
For the Label, I created the following field
[Selected Metro Name]
IIF([Metro Name]=[Metro Name Parameter],[Metro Name],NULL)
brought this onto the Label Tile and set to All and 'Allow Labels to overlap other marks'...and voila the highlighting in the scatter is now controlled by the parameter.
Now I've not done it here, but you'll now need to use the parameter to set the filtering in the other sheets...you can actually just use the [Selected Metro 1/0] and set the filter =1 (and replace the filter with the parameter in the dashboard)
...bit of a work around, but you do want the same thing (the filter) to perform 2 different things at the same time!!
MAIN METRO MAP 072016 - SR.twbx 461.4 KB
Did you get any solution for this ? I have the same requirement.