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  • Ivan Petrov thinks this is interesting:

 tooltip : {
         useHTML: true,
         headerFormat: '<table><thead><tr>' + 
             '<th style="border-bottom: 2px solid #6678b1; color: #039" ' + 
             'colspan=2 >{point.key}</th></tr></thead><tbody>',
         pointFormat: '<tr><td style="color: {series.color}">' + 
             '<img src="./series_{series.index}.png" ' + 
             'style="vertical-align:text-bottom; margin-right: 5px" >' + 
             '{series.name}: </td><td style="text-align: right; color: #669;">' +
             '<b>{point.y}</b></td></tr>',
         footerFormat: '</tbody></table>'
      }

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Cover of Learning Highcharts

Note

pretty cool