code: function onmousemove(e) {
  			var point,
  				points,
  				hoverPoint = chart.hoverPoint,
  				hoverSeries = chart.hoverSeries,
  				i,
  				j,
  				distance = chartWidth,
  				index = inverted ? e.chartY : e.chartX - plotLeft; // wtf?
  
  			// shared tooltip
  			if (tooltip && options.shared) {
  				points = [];
  
  				// loop over all series and find the ones with points closest to the mouse
  				i = series.length;
  				for (j = 0; j < i; j++) {
  					if (series[j].visible && series[j].tooltipPoints.length) {
  						point = series[j].tooltipPoints[index];
  						point._dist = mathAbs(index - point.plotX);
  						distance = mathMin(distance, point._dist);
  						points.push(point);
  					}
  				}
  				// remove furthest points
  				i = points.length;
  				while (i--) {
  					if (points[i]._dist > distance) {
  						points.splice(i, 1);
  					}
  				}
  				// refresh the tooltip if necessary
  				if (points.length && (points[0].plotX !== hoverX)) {
  					tooltip.refresh(points);
  					hoverX = points[0].plotX;
  				}
  			}
  
  			// separate tooltip and general mouse events
  			if (hoverSeries && hoverSeries.tracker) { // only use for line-type series with common tracker
  
  				// get the point
  				point = hoverSeries.tooltipPoints[index];
  
  				// a new point is hovered, refresh the tooltip
  				if (point && point !== hoverPoint) {
  
  					// trigger the events
  					point.onMouseOver();
  
  				}
  			}
  		}
docs: With line type charts with a single tracker, get the point closest to the mouse