Adam Evans,
Updating legendText on hovering over the datapoint should work fine as per your requirements. Please find the code-snippet below.
mouseover: onMouseover(e){
e.dataSeries.legendText = (e.dataSeries.dataPoints[e.dataPointIndex].x + ": " + e.dataSeries.dataPoints[e.dataPointIndex].y).toString();
e.chart.render();
}
Please take a look at this JSFiddle for an example of the same.
—
Vishwas R
Team CanvasJS