score:20

Accepted answer

The code below will handle one or more tooltips. 

function openTip(oChart,datasetIndex,pointIndex){
   if(window.oChart.tooltip._active == undefined)
      window.oChart.tooltip._active = []
   var activeElements = window.oChart.tooltip._active;
   var requestedElem = window.oChart.getDatasetMeta(datasetIndex).data[pointIndex];
   for(var i = 0; i < activeElements.length; i++) {
       if(requestedElem._index == activeElements[i]._index)  
          return;
   }
   activeElements.push(requestedElem);
   //window.oChart.tooltip._view.body = window.oChart.getDatasetMeta(datasetIndex).data;
   window.oChart.tooltip._active = activeElements;
   window.oChart.tooltip.update(true);
   window.oChart.draw();
}

function closeTip(oChart,datasetIndex,pointIndex){
   var activeElements = window.oChart.tooltip._active;
   if(activeElements == undefined || activeElements.length == 0)
     return;
   var requestedElem = window.oChart.getDatasetMeta(datasetIndex).data[pointIndex];
   for(var i = 0; i < activeElements.length; i++) {
       if(requestedElem._index == activeElements[i]._index)  {
          activeElements.splice(i, 1);
          break;
       }
   }
   window.oChart.tooltip._active = activeElements;
   window.oChart.tooltip.update(true);
   window.oChart.draw();
}

Complete solution provided by @BeetleJuice - https://jsfiddle.net/ucvvvnm4/5/

score:21

I would be careful accessing/modifying private variables that begin with _. You may find yourself with unexpected behavior. Why not trigger the canvas mousemove event

  function openToolTip (myChart, index) {

    var mouseMoveEvent, meta, point, rectangle, value;

    meta = myChart.getDatasetMeta(0);
    rectangle = myChart.canvas.getBoundingClientRect();
    point = meta.data[index].getCenterPoint();

    mouseMoveEvent = new MouseEvent('mousemove', {
      clientX: rectangle.left + point.x,
      clientY: rectangle.top + point.y
    });
    
    myChart.canvas.dispatchEvent(mouseMoveEvent);

  },

To close the tooltip just trigger the mouseout event

  function closeToolTip (myChart) {

    var mouseOutEvent = new MouseEvent('mouseout');
    return myChart.canvas.dispatchEvent(mouseOutEvent);

  }

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