In Blox Plot, points are showing as dimension & Action filter is always on dimension not on aggregation measure. Here is the Idea section where you can vote for this: - http://community.tableau.com/ideas/2399
Here one similar problem with solution I have found: - Measure Names in table and action filtering via Link
Hope these may help!
Is there supposed to be a relationship between these two box plots, or are you just trying to create a link to another sheet (in other words, are you trying to link these based on common fields, or just trying to create a navigation link to another sheet?) If the two are not related, there may be a workaround for initiating an action in the "box" area as well as the data points.
I cannot get it working either way. I was thinking you could place a transparent image as a shape on a sheet, and float that over the box plot, and use it to trigger an action to take you to the next sheet (again, this would assume we aren't linking the sheets based on common dimensions--just simply creating a way to navigate from one sheet to another). However, when you place a transparent image on a sheet as a Shape, and float it over the box plot, the image/sheet is no longer transparent--so, I could not get it to work for this.
If we could trigger actions from Image objects, this would be possible--or if we could trigger an action with a blank sheet with no fields on it (or simply make a sheet completely transparent) that would work too, because blank sheets can be floated over marks and they show up transparent.
Unfortunately, I couldn't seem to make this work--although I thought I had seen a similar approach somewhere recently.