If you add an ID to the panel "bubblechart", then add this as a script to the dashboard, it'll re-write the epoch time strings to a date format. Verified working on Splunk 6.4.4. 
  require(['jquery', 'splunkjs/mvc/simplexml/ready!'], function($) {
        setTimeout(function(){
                $('#bubblechart').find('.highcharts-xaxis-labels').children('text').each(function(){
                        var epochtime = parseInt($(this).html().replace(/\,/g,''),10);
                        var objDate = new Date(epochtime*1000);
                        $(this).html(objDate.toLocaleDateString('en-US'));
                });
        }, 1000);
});
  
						
					
					... View more