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);
});
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