score:0

Accepted answer

you can enable the usehtml option and style the tooltip by css:

.highcharts-tooltip > span {
    padding: 10px;
    border: 2px solid red;
    border-radius: 30px 30px 30px 0;
}

live demo: http://jsfiddle.net/blacklabel/wver5hyf/

about border radius: https://www.w3schools.com/cssref/css3_pr_border-radius.asp


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