I’m pretty sure, that similar topic exists on stack, however I can’t find it. In general it is possible to achieve that using HTML tooltip, see: http://jsfiddle.net/7wVDV/147/
CSS:
.highcharts-container {
overflow: visible !important;
}
.MyChartTooltip {
position: relative;
z-index: 50;
border: 2px solid rgb(0, 108, 169);
border-radius: 5px;
background-color: #ffffff;
padding: 5px;
font-size: 9pt;
}