In regards to selecting multiple filter values, you have two options:
1. You can use hold down the 'control' key to select multiple values, this will allow you to pick multiple SMA Ratings to filter on (so oldest + youngest would work).
2. Instead of using a dashboard action to filter on the SMA Rating size chart, you could instead build a filter on the worksheet and use that for filtering. I've attached a copy of the workbook showing what I did to achieve that.
As to your second question, I'm not sure we can have Tableau automatically retain the large size when you've filtered down to only one age group. Someone else may be able to give you more guidance there though.
Hope this helps!
You may want to use a Continuous (Green) Pill for the Size.
With that you'd have an option to use a fixed range of sizes
when editing the Size Legend for that Pill.
Please find the attached.
Hope it could help.
Thanks Cheryl, Yuri,
In following your instructions I somehow stumbled onto a solution that works really well. I added a filter to the dashboard that filters the graph by my two variables. I then added a highlight component as an action in the dashboard. This makes it so that when I select a subset of data to view, it only highlights that data and generates a new trend line. This keeps all the data points the same size regardless of what subset it is highlighted!
Thanks again for your help. I'm sure I'll get a better handle on all of these tools as I continue to use Tableau.