Accepted answer

Everytime you add a point, I'd check if the 1st point was < however far back you want to go. If it is, remove it.

if (chart.series[0].data[0].x < time cutoff) {


Since you're doing a consistant time interval, you should be safe with just checking the 1st point.

Another option based on the comments above:

var bShift = false;
if(!bShift && chart.series[0].data[0].x < time cutoff)  
    bShift = true;
chart.series[0].addPoint(point, true, bShift);

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