Re-render Widget after column number changes
I found an answer that works for me at least..
Since I render my widget via Javascript and not via PHP/Twig I can use this

$(document).on("click", ".colspan-picker a", function(){
    var container = $(this).parent().parent().parent();
    widgetID = container.data("id");
    sRGoogleAnalyticsRerenderChart(widgetID);
});


.colspan-picker a is the element for columnchange and its parent().parent().parent() has a data-id of the widget container