The Polis embed code and <iframe/>
are configured to emit a "vote" event each time a participant votes.
You can subscribe to those events on the page that instantiates the <iframe/>
like this:
window.addEventListener("message", function(event) {
var data = event.data||{};
if (!event.origin.match(/pol.is$/)) {
return;
}
if (data.name === "vote") {
alert("vote!");
}