Highcharts spline dataLabels overlapping
You can use that plugin for repositioning dataLabels: http://jsfiddle.net/menXU/1/ It's not perfect, since works only for max 2 series and requires disabled animations, or you will see little delay when repositioning dataLabels.
How to use? Copy StaggerDataLabels and isLabelOnLabel functions, and then use StaggerDataLabels in load and redraw events for chart.