Paul,

This may not have the aesthetic or functionality that you need,
but maybe it could be a first step.

My data set was of the form: Country, VotedFor.

I created a dual axis map.

One axis was just a grey circle on each [Country].

The second axis was a filled map with [VotedFor].

In this scenario, when I clicked on a [Country]'s grey circle,

the countries that [VotedFor] it were highlighted.

But the clicking-on and clicking-off is clunky.

Likely something much better could be done

with  highlight /filter dashboard actions.