score:2

Accepted answer

It worked for me when i changed:

function mouseOverCallback(event, series) {
  series.graph.attr('stroke', 'steelblue')
  series.graph.toFront()
}

to

function mouseOverCallback(event, series) {
  series.graph.attr('stroke', 'steelblue')
  series.group.toFront();
}

score:1

You could try simply updating the zIndex of the series upon mouseOver and mouseOut.

For example (JSFiddle):

plotOptions: {
    series: {
        stickyTracking: false,
        events: {
            mouseOver: function (e) {
                e.target.update({ zIndex: 1000 });
                mouseOverCallback(e, this)
            },
            mouseOut: function (e) {
                e.target.update({ zIndex: undefined });
                mouseOutCallback(e, this)
            }
        }
    }
}

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